True Time. 69 



multiple of 2) cells is now arranged, so that the positive 

 pole of one cell is connected with A, and the negative of the 

 other with B\ the other positive and negative poles are 

 joined together and connected with ft Now as the clock 

 goes, and the springs A and B are alternately brought into 

 contact with ft each second, an alternately positive and 

 negative current will flow along the wire to ft Any length 

 of wire may now be interposed between the contact piece ft 

 and the united poles of the battery, and this wire will form 

 the time main. 



If we wish to lay on the supply to any other seconds 

 pendulum clock to control it, the following arrangements 

 have to be made. A pendulum bob, formed of a hollow 

 bobbin of covered copper wire, has to be fitted on to the 

 pendulum instead of the one hitherto used in the clock to be 

 controlled. The two ends of the copper wire are led insu- 

 lated up the pendulum-rod to the suspension-spring, where 

 they are coiled into spirals (so as to offer no resistance as the 

 pendulum swings) and made to terminate in two binding- 

 screws on the clock case. Two permanent magnets have now 

 to be fixed in such a way that as the pendulum swings the 

 hollow ends of the bobbin just enclose the ends of the 

 magnet at the end of each oscillation. The time main is 

 now led to one of the binding-screws holding one end of 

 the pendulum wire, and led on from the other screw to other 

 clocks or to the ground, which will form the return wire. 



Now supposing the circuit complete and the Observatory 

 clock going, it sends its alternating positive and negative 

 currents through ft along the time main and into the pen- 

 dulum, and by a well-known law converts the bob into a 

 magnet with two poles at every current, and as the currents 

 are alternately positive and negative, the poles of the bob 

 will also alternate. Suppose we set the pendulum swinging, 

 and it arrives at the end of its swing to the left with a 

 north pole, it meets with the N pole of the magnet, which 

 tends to repel it, while the opposite end of the bob being S* 

 is attracted by the right hand magnet as it swings to the 

 right. The right side of the bob is now N., and is again 

 repelled by the magnet on the right until, in a very few 

 seconds, it commences to swing so that the ends of the bob 

 are always S. as they approach the N. poles of the permanent 

 magnets ; and once oscillating in this manner, it is, so long 

 as the currents flow properly from the standard clock, a diffi- 

 cult thing to make the controlled clocks go any other way 



