to Practical Astronomy. 147 



From one pole of a three-celled battery a wire is led to a 

 terminal of the clectro-maguet of the clock circuit, and a 

 wire also connects the other terminal with the relay, where 

 it ends in a platinum point on the top of the armature lever ; 

 the other pole of the Imttery is connected directly with the 

 relay, and ends also in a platinum point, close and opposed to 

 the other. The circuit of this system is therefore incomplete 

 only by the separation of the two platinum points above men- 

 tioned, which are kept apart by means of a delicate spring. 

 The second circuit includes the clock contact apparatus, and 

 the delicate electro magnet of the relay, and may be thus 

 traced : — A pole of a single cell battery is connected by a 

 wire directly with one of two springs, which are a portion of 

 the contact apparatus of the astronomical clock; the other 

 pole is connected to one terminal of the relay magnet coil, 

 whose other terminal is connected by a wire with the second 

 spring of the clock contact; this circuit then is only incom- 

 plete by the separation of the two springs. 



The '^^ clock contact'^ is thus arranged: — An extra wheel of 

 sixty conical teeth is fixed in the escape wheel arbor of the 

 astronomical clock; a delicate spring, Avith a jewelled pallet, is 

 so placed tliat Avhen the clock is at rest the pallet drops into 

 the angle Ijetween the teeth; at the back of this spring is a 

 little platinum stud; another spring, with a platinum stud, is 

 placed just behind this, and is so adjusted that a slight lift- 

 ing of the front spring brings the two platinum studs into 

 contact. 



"When the clock is going, the escape wheel revolves once in 

 a minute, and the extra Avheel moves one tooth each second, 

 which in passing presses against the pallet, and lifts the spring 

 just sufficiently to bring the studs into contact for an instant, 

 and so on for each second. One of the teeth of the contact 

 wheel is filed down, so that it passes without lifting the 

 spring, and the seconds' hand is so placed that it points at 60 

 on the dial Avhen this short tooth comes to the pallet. The 

 two springs are firmly but separately attached to a block of 

 insulating material, which is itself screwed to the front plate 

 of the clock, so that the springs are insulated from the clock 

 works and from each other. There are screw adjustments 

 for regulating the lift of the front spring and the distance 

 between the studs. 



Tlie object of introducing a relay magnet into the clock 

 circuit is to avoid thcoxydizationof the platinum studs in the 

 clock contact, which would result from using directly a battery 



L 2 



