Largeimage(99 kB).Fig. 2078Fig. 2078.
Largeimage(99 kB).
Fig. 2078.
The wheel to be cut is carried as follows: Upon the bed-plate of the machine is placed a headb,Fig. 2078, corresponding to the headstock of a lathe, opposite which is a headb′, answering to the tailstock of a lathe. These two carry a mandreld, to which is fastened a face-plated′against which the work is chucked. At the end ofd′′is fixed, in the usual manner, the worm-wheel for the dividing mechanism. The cutting arbor is held in a head that is carried in a cross slidec2,Fig. 2077, this cross slide being a carriage that may be fed along the side extension of the bed, which is broken off in the plan view of the machine,Fig. 2078. The two slides thus provided in this machine form in effect a longitudinal and cross feed, answering to the feeds of a lathe carriage and tool rest.
The cutter headm,Fig. 2077, is composed of two parts,candm. Provision is made to swing the head in two directions, one ofwhich is noted by the plain arrow and the other by the feathered arrow in the engravings. Between the two the cutter arbor, it will be perceived, may be set at an angle in whatever direction the nature of the work may require. Referring toFigs. 2076and2077, it will be seen that the cutter-driver mechanism operates as follows: The tight pulleya1, driven in the direction noted by the arrow, turns the conea6which drives the pulleyb. The belt frombpasses over grooved idlers,b1,b2,b3, &c., to the grooved pulleyb8, which is fast on its shaft and drives a train of gearing that operates the cutter arbor, the train being best shown inFig. 2077. The train of gearing thus driven is composed of gearsc1,c2andc5, the latter being on the cutter arbor. The object of this arrangement is to obtain a high belt velocity. It will be seen that all these gears have their teeth at an angle to their axes, a feature that has been introduced to obtain smoothness of action. To maintain equal tension of belt at whatever angle the cutter may be set, the idle pulleyb2acts as a belt tightener, being carried by the rodstandt1.
Fig. 2079Fig. 2079.
Fig. 2079.
Referring now to the feed motions, the machine is provided with a quick return for the cutter, the mechanism of which is as follows: The cone pulleya4,Fig. 2077, is mounted upon a driver shaftd,Fig. 2079. Upon this shaft are two loose bevelled pinionsd2d4, between which, and splined to the shaft, is a clutchf. For the feed traverse the clutchfis moved to engage with the piniond4, while for the quick return it engages withd2. This device corresponds to the old-style quick-return motion used in some of the heavy English planing machines. The clutchfis operated by a rodl′, and drives the bevelled pinionsd2d4by friction. The hub of the clutch is coned to fit a coned recess in the hubs of the two pinions. A pair of gears,d6d7, transmit the motion ofd5to the shaftd1, on the end of which is the pinione1, Motion is conveyed from this pinion to the feed-screwe,Fig. 2081, by the intermediate gears e2,e3,e4ande5, and also by the helical pinionse6ande7, the latter two being also shown inFig. 2081.
Fig. 2080Fig. 2080.
Fig. 2080.
Referring to the dividing mechanism,e,Fig. 2077, is an index-wheel operated by a worm.e1is an arm with a locking tongue. Motion fromeis conveyed to the shaftgthrough a swing-frame, shown in dotted lines inFig. 2077, and a train of gearsg2,g3,g4,g5,g6. On shaftg,Fig. 2078, is a pair of angular-toothed beveled pinions,h1h2, and on shafth,Fig. 2080, is a pinionh3, driving a pinionh4, which in turn drives pinionsi i1. The latter drive the wormh′which operates the wheelh. The two shafts carryingi i1are supported by a piecef, the arm of which appears in section. This is fixed on the large toothed wheelg, indicated by the dotted lines in the same figure. The piecefabove referred to is not fully shown in the engraving, portions of it having been omitted in order to show the mechanism previously mentioned. The wheelhis mounted on shaftd′′, and is used to revolve the face plated′, all as shown inFig. 2078. The wheelsg2g3are change wheels, whose relative diameters determine the number of turns the wheelemust make for a givenpitch. The arme1,Fig. 2077, is provided with a spring to hold the index pin into the notch of the index wheel. From this description it is obvious that when the number of the teeth of the wheel to be cut is a multiple of that of the wheelh, the number of turns to be given to the tangent screwh′,Fig. 2080, is exactly determined by the ratio existing between these two numbers. On the other hand, where the number of teeth required is not a multiple of the teeth in the wheelh, the number of turns to be given to the screw will be equal tonplus a fraction. In the first case, if all the intermediate gears between the dividing apparatus and the tangent screw are arranged to transmit to the former a number of definite turns, it will suffice to make the crank describe the number of turns indicated by the ratio the wheelebears to the worm-wheel. In the second case, in order to give the tangent screwnturns plus a fraction by giving the crankn+ turns, it is necessary to employ several wheels, for which the ratio must be calculated. If the division so obtained is not an exact divisor of the number of teeth of the wheelh, it is necessary that one of the wheels forming the combination shall have a number of teeth which is a multiple of the division mentioned.
Another consideration with reference to the number of turns to be given to the crank of the dividing apparatus is mentioned in the inventor’s description of this machine. The smaller the number the greater will be the chance of error in the result; for example, if it be supposed that a division corresponding to one turn of the tangent screw is to be made, if only one turn of the crank is made, the play unavoidable where easy movement is secured will be repeated and multiplied in the same way that an error is produced after a certain number of divisions. If, on the contrary, the mechanism be arranged so that the number of turns of the crank is multiplied in obtaining one turn of the tangent screw, the error will be appreciably reduced. It is therefore recommended by the designer of this machine to arrange the train of gears so as to give a certain number of full turns to the crank in all cases. If, after having cut the teeth in the blank, it is desirable to go over them again, it is simply necessary to turn the screwjwhich engages with the gear-wheelj1.
Fig. 2081Fig. 2081.
Fig. 2081.
The next feature to be described is the adjustment of the cutter. In some cases it is necessary to incline the cutter in such a way that the axis of the shaft carrying it forms a certain angle with the vertical. This is the case in cutting angle teeth, as shown inFig. 2076. In order to produce the necessary angle for such teeth, it is only necessary to turn the wormkthat engages with the worm-wheelk1,Fig. 2077. This wheel is fast on to the piecem, and the latter, when set to the desired inclination, is kept in place by means of boltso,Figs. 2077and2081. In some cases it is necessary to incline the cutter in such a way that theaxis of the shaft that carries it does not cease to be in a vertical plane perpendicular to the shaftd, this being the case as illustrated inFig. 2082. In order to obtain this obliquity the small shaftmis turned, and the movement so obtained is transmitted by means of two small pinionsm2m3to the shaft carrying at its extremity the screwn′. This screw gears with the segmentn′′. The latter is fixed to a piecej, furnished with bearings for the reception of the shaft that drives the cutter spindle, which is adjusted endways by means of the nuts shown.
Fig. 2082Fig. 2082.
Fig. 2082.
If it is desired to produce a wheel with angle teeth it is necessary, after having arranged the cutter as shown inFig. 2076, and while the forward motion of the carriage takes place, that the wheelrshall turn with a slow, regular movement until the tooth operated upon is finished. After this the tool retraces its path at a somewhat higher speed. This automatic motion is obtained from a shaft (Fig. 2076), on which are placed the pinionse2e3. This shaft carries a third pinionp2, which, by means of one or more pairs of wheels mounted two by two on a swinging framep, as shown byp3p4p5, turns the shaftp′(Fig. 2080), which carries at one of its extremities the wheelp5and at the other the screwh3. This screw, by proper intermediates, operates the toothed wheelg,Fig. 2080, which in its rotation carries along the piecef, with all the parts supported by it. In this movement the pinionh3does not turn, nor does the second pinionh4, which slides on the former. The screwh′slightly turns the large wheelh, which, as previously mentioned, is mounted on the shaftd,Fig. 2078. When the special tooth operated upon is finished the movement is reversed by operating the leverl. The table and the wheelr,Fig. 2077, then move in the opposite direction. When the original position is reached by the cutter, the reversing lever is thrown out of gear; the handlee′is then used so as to effect the proper division, and the machine is again started.
As has been shown, only a small portion of the circumference of the wheelgis subjected to wear. In this way it would be possible to limit the operation of cutting the teeth to a certain length of arc only. In that case, however, considerable wear would be produced; for this reason the constructor has preferred to provide the whole circumference with teeth, in order to change the working point from time to time, so as to distribute the wear. In order to permit this displacement it is necessary to disengage the wormk(Fig. 2076), which is accomplished by turning the hand wheelv, mounted on the shaftv′,Fig. 2078. This shaft carries at each extremity small pinions,v2,v3, gearing with other pinions fixed at the extremity of each of the supports of the shaftp′.
In order to make the operation of this machine better understood, we will conclude our description by some practical examples of the calculations required in making helical teeth. It will be observed that the two small movements necessary in cutting an angle tooth in a given inclination are obtained first by the screwe,Fig. 2077, feeding the cutter head, and second by the tangent screwk,Fig. 2076, that governs the rotary motion of the wheelg, and consequently of the shaftd, carrying the face plate and the blank to be cut. The second wheelh, mounted on this shaft, is driven by the endless screwh′,Fig. 2080, the supports of which are fixed on the wheelg. It will be observed at the same time that the speed of the screweacting upon the tool holder is the same as that of the shaft carrying the wheelse2e3andp2, since the wheelse4e5e6e7have the same number of teeth. It is obvious, therefore, that that ratio of speed which will exist between the tangent screwkand the shaft of wheelse2e3andp2will have to be the same as that between the driving screweof the cutter head and the tangent screwk. Consequently, the combinations of wheels that connect this tangent screwkto the shafte2e3andp2will produce the same effect as if they were connected directly with the feed screwe. This being established, the general formulæ determining the gearing to be employed in order to produce helical teeth inclined at a certain angle are obtained in the following manner: It should here be observed that the teeth produced will be what in the United States are called angle teeth, corresponding, however, so nearly to the helix as to be considered helical. Suppose that the number of teeth in the wheelgis 300, and that the pitch of the driving screw of the cutter head is 5 mm., using for convenience the French system of measurements. Letx/ybe the ratio of the four wheels that it is necessary to mount. Letmdesignate the degrees of inclination of the teeth. Letpequal the pitch of the desired helix, anddthe diameter of the wheel to be operated upon. We then have cotan.m=p/(d× 3.14), from which we findp= cotan.m×d× 3.14, and in order to make the cutter head run over a distance corresponding to this pitch, the driving screwemust make a number of turns equal to
But while the cutter head passes over a distance equal to thepitch, the wheelgmakes one turn and the tangent screw 300 turns; consequently, the ratio to be established between the speed of the tangent screw and between that of the screw driving the carriage will be represented by
Thus, for a wheel with a diameter of 1.75 inches, the machine ought to have an inclination of 15° to the primitive circumference, and we would have, for the ratio to be established between the tangent screw and the driving screw,
It should be remarked that, according as the angle should be either to right or to left, one or two intermediate pieces are placed on the swing-frame, the slide of which is nearly horizontal. The speed of the driving shaft, supported by the column mentioned in introductory remarks, is 120 revolutions; that of cutter equals from 20 to 30 revolutions; that of screw of cutter head, advance from 1 to 42 revolutions, return from 7 to 66 revolutions.
Vice work may be said to include all those operations performed by the machinist that are not included in the work done by machine tools. In England vice work is divided into two distinct classes, viz., fitting and erecting. The fitter fits the work together after it has been operated upon by the lathe planer and other machine tools, and the erector receives the work from the fitter and erects it in place upon the engine or machine. Fitting requires more skill than turning, and erecting still more than fitting, but it is at the same time to be observed that the operations of the erector includes a great many of those of the fitter. In treating of the subjects of vice work and erecting, it appears to the author desirable to treat at the same time of some operations that are not usually included in those trades, because they are performed with tools similar to those used by the fitter, and may be treated equally as well in this way as in any other, while a knowledge of them cannot fail to be of great service to both the fitter and erector. Among the operations here referred to are some of the uses of the hammer; such, for example, as in straightening metal plates.
The vice used by the machinist varies both in construction and size according to the class of work it is to hold. For ordinary work the vice may possess the conveniences of swiveling and a quick return motion, but when heavy chipping constitutes a large proportion of the work to be done the legged vice is preferable.
The height of vice jaws from the floor is usually greater for very small work than for the ordinary work of the machine shop, because the work needs to be more clearly observed without compelling the operator to stoop to examine it. The gripping surfaces of vice jaws are usually made to meet a little the closest at the top, so as to grip the work close to the top and enable work cut off with a chisel to be cut clean and level with the jaws.
Fig. 2083Fig. 2083.
Fig. 2083.
The jaws of the wood-worker’s vice are made then as inFig. 2083, and reach higher above the screw than the vices used for iron work, because the work is often of considerable depth, and being light will not lie still of its own weight, as is the case with iron.
Fig. 2084Fig. 2084.
Fig. 2084.
An example of the ordinary vice of the machine shop is shown inFig. 2084, which represents partly in section a patent swivel vice.ais the jaw in one piece with the body of the vice, andbis the movable jaw, being the one nearest to theoperator. The movable jaw is allowed to slide freely through the fixed one (being pushed or pulled by hand), or is drawn upon and grips the work by operating the handle or leverh. The means of accomplishing this result are as follows: As shown in the cut,bis free to be moved in or out, but ifhbe pulled away from the vice, the shoulderc, meeting the shouldern, will move the toggleg, and this, through the medium ofg′, moves the tooth bart, so as to engage with the teeth on the side of the movable jaw bar shown att. As soon as the teethtmeet the teethtthe two travel together, and the jawbcloses on and grips the work. But as the motion is small in amount, the jawbshould be placed so to nearly or quite touch the work beforehis operated. To unloose the work, the handlehis operated in an opposite direction, and the hookmmeetsmand pullstto the position shown. The springsoperates upon a hook atu, to engage the teetht, with the rackt, as soon as the handlehis moved in the tightening direction. The vice grips with great force, because during the tightening the toggle,gis nearly straight, and its movement less than would be the case with a screw-vice having the ordinary pitch of thread and under an equal amount of handle movement.
Fig. 2085Fig. 2085.
Fig. 2085.
In this vice the fixed jaw is made to fasten permanently to the work bench, but in others having a similar tightening mechanism the fixed jaw is so attached to the bench as to allow of being swivelled. The method of accomplishing this is shown inFig. 2085, in whichsis the foot of the vice bored conical to receive a cone on the castingr, which is fastened to the benchb.wis a washer andhthe double arm nut. Loosening this nut permits of the vice being rotated uponr.
When handlehis operated to release the movable jaw it can be moved rapidly to open and receive the work, and to close upon the work, when by a second handle movement the work can be gripped, the operation being much quicker than when the movable jaw is traversed by a screw and nut.
In this vice the gripping surface of the jaws are always parallel one to the other, and attachments are employed to grip taper work as wedges.
Fig. 2086Fig. 2086.
Fig. 2086.
Fig. 2087Fig. 2087.
Fig. 2087.
InFig. 2086is represented a patent adjustable jaw vice, which is also shown inFig. 2087with the adjustable jaw removed and upside down. From the construction it is apparent that the grooveg, being an arc of a circle of whichcis the centre, the jaw is, as it were, pivoted horizontally, and can swing so as to let the plane of the jaw surfaces conform to the plane of the work; hence a wedge can be gripped all along the length enveloped by the jaws, and not at one corner or end only. When the pinais inserted the jaw stands fixed parallel to the sliding jaw. The pinaengages in a ratchet in the base below it to secure the back vice jaw in position when it is set to any required angle.
A second convenience in this vice is that the whole vice can be swivelled upon the base that bolts to the bench, which is provided with a central hole and annular groove into which the base of the field jaw pivots; atbis a spring pin passing into holes in the bench plate, so that by lifting the pinb, the whole vice can be swung or rotated upon the base or bench plate, until the pinbfalls into another hole in the base plate, which is provided with eight of these holes. The movable jaw is here operated by a screw and nut.
Fig. 2088Fig. 2088.
Fig. 2088.
Fig. 2088represents a form of leg vice for heavy work. In the ordinary forms of this class of vice the two gripping surfaces of the jaws, only stand parallel and vertical when at one position, because the movable leg is pivoted atp; but in that shown in the figure the movable jaw is supported by the arma, passing through the fixed legl, which carries a nutn. A screws, having journal bearing in the movable leg, screws through the nutn, and is connected to the upper screw by the chainc, which passes around a chain wheel provided on each screw, so that the movable leg moves in an upright position and the jaw faces stand parallel, no matter what the width of the work. This is a very substantial method of obtaining a desirable and important object, and greatly enhances the gripping capability of the vice.Fig. 2089represents a sectional view of another patent vice.ais the sliding andbthe fixed jaw.pis the bed plate carrying the steel rack plateh. Attached to each side of the base of the handle is a disk. These disks are carried on the outer end of the movable jawa, and are held in place by the friction strapst, adjusted by the screwss. On the radial face of the disk is the pink, which, when the handle or lever is lifted or raised, depresses the end of leverj, which at its other end raises the clutchg, disengaging the same from the rackh, as shown in the engraving. The jawais thus free to be moved byhand, so as to have contact with the work. To tighten the vice the handle is depressed, whereonkreleasesjand the latter permits the toothed clutchgto engage with the teeth ofh. At the same time the bard, which is pivoted to the disks, is drawn outward. The end of the bard, meeting the surface of the lug shown ona, acts (in conjunction with the toothed clutchh) as a toggle fulcrum from which the disks may force the movable jaw to grip the work.
Fig. 2089Fig. 2089.
Fig. 2089.
This action may be more minutely described as follows: The enddofdis pivoted upon the disks, as shown; hence when the handle is depressed the effort of the enddis to move to the right, butdbeing fixed at the other end the pressure is exerted to force the movable jaw to the left, and therefore upon the work. The amount of jaw movement due to the depression of the handle is such that if that jaw is pushed near or close to the work the handle will stand about vertical downward when the vice firmly grips the work.
For vices whose jaws cannot be swiveled horizontally to enable them to conform to taper work, attachments for the jaws are sometimes provided, these attachments having the necessary swiveling feature. So likewise for gripping pipes, and similar purposes, attachments are made having circular recesses to receive the pipes.
Fig. 2090Fig. 2090.
Fig. 2090.
Fig. 2091Fig. 2091.
Fig. 2091.
To prevent the vice jaws from damaging the work surface, and also to hold some kinds of work more firmly, various forms of clamps, or coverings for the vice jaws are used. ThusFigs. 2090and2091represent clamps for holding round or square pins. In the former the grooves pass entirely through the clamp jaws, so as to receive long pieces of wire, while in the latter the recesses are short, so as to form an abutment for the end of the pins, and act as a gauge in filing or cutting them off to length.
Fig. 2092Fig. 2092.
Fig. 2092.
An excellent form of pin clamp is shown inFig. 2092, the spring bow at the bottom acting to hold the jaws open and force the faces against the vice jaws when the latter are opened. The flanges atb brest upon the tops of the vice jaws; hence it will be seen that the clamp is not liable to fall off when the vice is opened to receive the work, which is placed either in the hole ataor that atb, as may be most desirable.
Fig. 2093Fig. 2093.
Fig. 2093.
Fig. 2093shows such a clamp holding a screw, the clamp jaws being forced against the screw by the vice jaw pressure, when the vice jaws are opened the spring of the bow will cause the clamp jaws to open and release the screw.
Clamps such as shown inFigs. 2090and2091, but without the pin holes, are also provided, being made one pair of copper and another of lead, the latter being preferable for highly finished work. As the filings are apt to imbed in the copper, and, furthermore, as the copper gradually hardens upon its surface, the copper clamps require to be annealed occasionally, which may be done by heating them to a low red heat and dipping them in water. Lead clamps will hold small work very firmly, and are absolutely essential for triangular or other finished work having sharp corners, and also for highly finished cylindrical work, which may be held in them sufficiently firmly to be clipped without suffering damage from the vice jaws. A piece of thick leather, such as sole leather, also forms a very good clamp for finished work, but to prevent its falling off the vice jaws it is necessary to cut it nearly through on the outside and at the bent corner.
The hammer in some form or other is used in almost all kinds of mechanical manipulation, and in each of these applications it assumes a form varied to suit the nature of its duty, and of the material to be operated upon. In the machine shop it is used to drive, to stretch, and to straighten.
The most skilful of these operations are those involving stretching operations, as saw and plate straightening, examples of which will be given.
In using a hammer to drive, the weight and velocity of the hammer head are the main considerations. For example, the force of a blow delivered by a hammer weighing 1 lb., and travelling 40 feet in a second, will be equal to that weighing 2 lbs, andtravelling 20 feet in a second; but the mechanical effects will be different. If received on the same area of impact the effects will sink deeper into the metal with the greater velocity, and they will extend to a less radius surrounding the area of impact. Thus in driving out a key that is fast in its seat, a quick blow is more effective than a slow one, both being assumed to have at the moment of impact an equal amount of mechanical force stored up in them. On the other hand, for riveting the reverse will be the case. In the stretching processes the hammer requires to fall with as dead a blow as possible. Thus the hammer handle is, for saw stretching, placed at such an angle to the length of the hammer that the latter stands about vertical when the blow is delivered. In straightening, the blow is varied to accommodate the nature of the work; thus a short crook or bend would be best straightened by a quick blow with a light hammer, and a long one by a slower blow with a heavier hammer, which would cause the effects of the blow to affect a greater radius around the part receiving the impact.
As an example of the difference in mechanical effect between a number of blows aggregating a given amount of energy and a single blow having an equal amount of energy, suppose the case of a key requiring a given amount of power to start it from its seat, and every blow delivered upon it with insufficient force to loosen its hold simply tends to swell and rivet it more firmly in the keyway.
Probably the most expert use of the hammer is required in the straightening of engravers’ plates, as bank-note plates; and next to this comes the ornamental repoussé work of the manufacturing jeweller.
The most expert hammer process of the machine shop is that of straightening rifle barrels and straightening saws and sheet metal plates.
In straightening rifle barrels, the operator is guided as to the straightness as follows: A black line is drawn across a piece of glass elevated to the light, and the straightener looks through the bore at this line, which throws a dark line of shadow along the rifle bore. If this line appears straight while the barrel is rotated the bore is straight; but if the line waves the barrel requires straightening, the judgment of the operator being relied upon to determine the amount of the error, its location, and the force and nature of the blow necessary to rectify it.
The following information on the duration of a blow is taken fromEngineering, the results having been obtained from some experiments by Mr. Robert Sabine. These experiments, which were intended as preliminary to a more extended inquiry, were made with a view to find approximately how the duration of a blow varied with the weight of the hammer, its velocity of descent, and with the materials. An iron ball weighing1⁄4lb. was suspended by a fine wire from an insulated support upon the ceiling; so that when it hung vertically it just grazed the vertical face of an ordinary blacksmith’s anvil placed upon its side on a table. By raising the ball and letting it swing against the face of the anvil a blow of varying force could be struck. On rebounding, the ball was arrested whilst the excursion of the galvanometer needle was observed. By measuring the angle through which the ball was separated, its vertical fall and final velocity could be easily deduced. In this way the greatest vertical height from which the iron ball was let fall on to the face of the iron anvil was 4 ft., the least about1⁄80inch. Six readings were taken for each height, and they were invariably found to agree amongst each other. The averages only are given in the following records:
From this it would appear that when the velocity of a blow is increased, the duration is decreased within a certain limit; but that it reaches a minimum. The velocity of impact in the first experiment was about sixty times as great as in the last one; but the duration of the blow appears to be reduced only to about one-fourth of the time. The blows given by two hammers of different weights were compared. No. 1 weighed 4 ozs., No. 2 weighed only 21⁄4ozs. The durations of the blows were as follows:
It appears from this that a heavier hammer of the same material gives a longer duration of blow.
In the course of these experiments it was observed that the ball after striking the anvil rebounded irregularly, sometimes to a greater, at others to a less height, and that some relation appeared to exist between the heights to which the ball rebounded and the excursions of the galvanometer needle due to the residue of the charge.
In the next series, therefore, the rebounds of the iron ball from the iron anvil were measured and recorded, from which it appeared that when the rebound was greater the duration of contact was shorter, andvice versâ.
The explanation of this is probably that when the energy of the blow is expended in bruising or permanently altering the form of the hammer or anvil by which the contact of the two is prolonged, it has less energy left to enable it to rebound, andvice versâ. Substituting a brass anvil and brass ball, it was found that the blow was duller, the rebound much less, and the duration contact nearly three times as great as when the iron ball and anvil were used.
This series also shows the longer duration of the blow when its velocity is small. Using a brass anvil and iron ball the duration of the blow was greater than when both were of iron, but less than when both were of brass.
Striking the brass anvil with a common hammer, the duration of the blow appeared shorter when struck sharply.
Striking the blacksmith’s anvil with a common carpenter’s hammer, the duration appeared to be nearly constant.
It was, of course, necessary to allow in each case the hammer to rebound freely, and not to prevent it doing so by continuing to exert any pressure at the instant of the blow. When this condition was observed, it was invariably found that the harder and sharper the blow the shorter was its duration. It was also noticed that whenever the anvil gave out a sharp ringing sound, the duration of the blow was much shorter than when the sound was dull.
A very slight error would be introduced by reason of thermo-currents set up between the metals at the moment of the blow. By reversing the direction of charge of the accumulator, however, the effect from this cause was found to be quite inappreciable.