May 9, 1889] 



NATURE 



41 



is attached, by means of a double spiral spring, a gun- 

 metal casting, curved at its ends to allow of the easy 

 entrance of the arrow. The spring-jacks are arranged 

 so that they can be taken out or replaced in the conduit 



in a few minutes in case of any failure. The electric 

 resistance that they offer is much smaller than would have 

 been anticipated, the total measured resistance of the 

 entire line being but little higher than the calculated re- 



FiG. 4. — The Spring Jack. 



sistance of the insulated cable. This is probably due to 

 the surfaces of the spring-jacks being kept bright and 

 clean by the arrow constantly running through them. 



affected by the starting or stopping of any other, it is 

 necessary, with series working, that a constant current 

 should be supplied to the circuit. Now while it is pos- 



In order that the speed of any one car shall not be sible, by winding the field-magnets of a dynamo in a 



Fig. s.— The Statter Constant Current Dynamo, showing Regulator. 



particular way, known as "compounding," to cause it, 

 when running at a fixed speed, to supply perfectly con- 

 stant pressure to a circuit, no matter how the resistance 

 of the circuit may vary, no such solution has yet been 



practically attained when the supply conditions are con- 

 stant current. Hence some mechanical device is neces- 

 sary in the latter case, and the one employed with the 

 Northfleet dynamo is that due to Messrs. Statter, and 



