Imágenes de páginas
PDF
EPUB

out at the muzzle. The object of the flat disc was to prevent the projectile from running over the wire and prematurely cutting it before the projectile left the muzzle. The rifling of the gun was very thoroughly washed with water, and the projectile polished in a lathe until it was bright. The two line wires from the chronograph were joined respectively to the projectile and gun. To secure contact with the gun the rear sight seat was removed and its screw served as a binding post. A make device described in the previous paper was placed at a determined distance in front of the muzzle and the terminals of the chronograph also extended to it. The object of this additional circuit was to determine where the interruption of the current by the projectile in its passage occurs, whether near the seat, or near the muzzle. This may be done by estimating the time between the break and the succeeding make on the negative and noting the corresponding distance on the trajectory. This comparison indicated that the break occurred near the muzzle and metallic contact is maintained.

A single ring brush.

The possibility of utilizing a single ring brush according to the plan conceived seemed now established. This plan involved making the gun itself one terminal of the chronograph circuit, thus utilizing the connection between projectile and gun as one of the brushes. From the projectile the current passed along a wire, imbedded in the wooden rod and connected with all of the copper bands, to the single brush at the muzzle which formed the other terminal of the chronograph. As a single brush could now be used, the advantages of one in the form of a ring entirely encircling the rod were at once apparent, for no matter which way the centripetal force urges the rod, a good contact with the ring is always assured, and furthermore the same ring may serve as a guide for the rod in its passage out of the bore.

The accurate cylindrical rod.

Attention was next directed toward obtaining a perfectly true round rod. The necessity of this requirement may not appear so serious at first thought, but keeping in mind the high velocity of the moving rod, the case may be likened with advantage to that of a railroad train moving at the rate of a mile a minute upon poorly ballasted track, compared with the smooth gliding of a train at the same speed on a good road bed. The question of the most suitable material for a rod was considered. The great mass of a metal rod of suitable size and length, and the

difficulty of preparing insulating bands upon it, pointed to the use of wood as the preferable material, and finally a fine piece of light white pine was chosen as the kind of wood to be used. The great length of this rod which was only 15% inches in diameter, made it impossible to turn it when supported in the lathe by its extremities alone. An attempt to place a third support in the center caused much annoyance by chattering when run at a sufficiently high speed. Finally a special tool was made which would support the rod and at the same time cut it to a true cylindrical shape. An iron collar with a hole just equal to the diameter of the desired rod was supported from the tool rest, and a specially made knife screwed upon this collar with its cutting edge turned so as to cut the wood in front of the collar as it advanced, down to a size which would just fit the hole. As the tool advanced the rod was polished by the friction in this collar which left a perfectly smooth and accurately finished rod. Notches were then cut at the desired intervals to accommodate the copper bands, which must be flush with the surface of the rod, by simply lowering the knife and running the tool in the opposite direction along the rod. This was done so that the tool would never come upon a smaller portion of the rod, since it served for a support.

Copper conductors added to the rod.

When the rod was turned a groove was cut along its entire length to accommodate a copper wire to be buried in it. Thin copper strips inch thick were cut to the desired lengths, and each made just long enough to completely encircle the rod without overlapping. These strips were first rounded between rollers, then wrapped around the rod and drawn very tight and close by winding a leather strap around it and drawing taut. Each edge was then secured by driving small brads closely along its length near the seam. The imbedded wire was next soldered to each strip of copper to secure good contact. Between the copper bands, in addition to the wire being sunk beneath the surface of the wood, further guard against metallic contact with the brush as it passed through was afforded by filling the groove flush with the surface with sealing wax. The rod was next replaced in the lathe and polished to an accurate smooth surface. A view of the rod prepared for use is shown in Figure 6. The shrapnel was bored out from the front to the base part with an inch drill to compensate for the additional mass of the rod, and a wooden plug driven in to give a firm bearing surface for the base of the ballistic rod. A collar was turned upon the

nose of the shrapnel to receive the ballistic rod which was firmly screwed in position. The wire imbedded in the rod was securely fastened to the projectile. At first this contact was secured by simply screwing down the rod thus pressing the wire upon the shoulder. Later, when the supply of unloaded shrapnel was exhausted, and it became necessary to use common shell, this method of securing contact was no longer reliable, as there were no screw threads in the projectile. It was more labor to prepare one of these common shell, as the front portion had to be cut off to make a bearing shoulder, and a hole bored through to the central cavity to admit the wire. Advantage was taken of the base percussion fuse to ensure good electrical connection. The fulminate and plunger were removed and the cavity filled with mercury, into which the wire passed through the perforation in the vent, originally intended to admit the flame to the cavity. This arrangement of mercury cup contact, thus found already made, was as good as though especially designed for the purpose. After each rod and projectile were prepared they were carefully tested for good electrical contacts, since experience proved that contacts supposed to be perfect were sometimes defective, and neglect of this precaution would have lost much time.

Spacing of the copper strips.

By any "dynamical method" the observations give points along a space-time curve. Since the number of these points is necessarily limited, their value greatly depends upon their position along the curve. The ideal positions of these points would seem to be at regular intervals along the arc of the curve itself. The form of the space-time curve is known to be such that observations at equal time intervals more nearly conform to the ideal than at equal space intervals. This also has the practical advantage of using the chronograph itself under the most favorable conditions. The copper bands should therefore be of varying lengths, the shortest being at the point of the rod. Accordingly for a first trial these lengths were approximated to in a rough way by simply taking the space-time curve to be a parabola, and the nearness of the approximation may be seen by the location of the observed points on the space-time diagrams given with each shot. Each negative was examined before the intervals on the rod for the succeeding shot were determined. Naturally the first attempts had only a few long intervals along the rod, and these were made shorter and shorter in succeeding shots to determine how near together the records might be easily

[graphic][merged small][merged small]
« AnteriorContinuar »