The Isochroniser.—This is a device for ensuring the correct speed regulation of the driving motors, and is shown in detail in Fig. 48. It comprises two portions, one portion being rotated at a definite speed by electrical means, and the other portion rotated by the driving motor.
The main portion consists of a metal tube N, bushed at both ends, the bottom end of the tube being arranged to work on ball-bearings. An ebonite bush C carries three copper rings T, T1, T2, and the brushes R, R1, R2 are in electrical contact with them. The ebonite plate J, 31/2 inches diameter, is secured to the top end of N, and carries a contact piece Q, shown separate at E. As will be seen this is a block of ebonite with three contacts arranged on the top surface. The middle contact P is 1/64th of an inch wide, and the contacts P1
and P2 are placed on either side at a distance of 1/16 inch; the contact strips P1, P2 carry the brass pins D, which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting wire is carried from the contact P to the copper ring T, another from P1 to T1, and one from P2 to T2.
N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T1, T2, copper rings; C, insulating block; R, R1, R2, brushes; J, ebonite disc; Q, contact block; D, metal pins; O, pulley, P, P1, P2, contact plates; K, needle; Z, spring; W, steel rod; E, countersunk bearing.
The bushes S are bored a running fit for the steel rod W (shown separate at A), which is coned at both ends, and runs between two countersunk bearings, the bottom bearing E being fixed while
the top bearing (not shown) is adjustable. A needle K is fastened near the end of the rod W, and attached to this needle is the spring Z, which presses lightly but firmly upon the contact block Q. To provide a level surface for Z to work over, the spaces between the contact pieces are filled in with an insulating material, and the whole surface finished off perfectly smooth. The spring Z is 1/8 inch wide for portion of its length, but at the point where it presses upon Q it is reduced in width to 1/64th of an inch (see Fig. 48). The driving arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a grooved pulley, is run in bearings parallel with the shaft W, and is connected by suitable gearing to the shaft of the driving motor, so that the needle K makes one revolution in about 21/2 seconds. A belt passing over the pulleys connects the two shafts, and the tension of the belt is regulated by means of an adjustable jockey pulley.
The tube N, carrying the disc J, must be rotated at a fixed speed, and this is accomplished in the following manner. An ordinary electric clock impulse dial, actuated from a master clock, is connected by suitable gearing H, so that the tube N makes exactly one revolution in 2 seconds; it being possible to adjust an electric clock of the "Synchronome" type, so that it only gains or loses about 1 second in 24 hours, and this provides
an accuracy sufficient for all practical purposes. The connections are given in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that a connecting wire is run from the steel spindle W to one terminal each of the lamps L, L1, L2, and from the other terminal of the lamps to one terminal of the batteries J, the battery comprising a set of three 4-volt accumulators. The other terminals of the batteries are joined one to each of the brushes R, R1, R2.