160 SUBMAKINE CABLE LAYING AND REPAIEING. 



The bars at one end make continuous rubbing contact with 

 the line and earth terminals in all positions of the key, while 

 at the other end they travel over a series of contacts. In all 

 there are five contacts, of which the two external ones are to 

 zinc, the central one to copper, and the intermediate ones to 

 two terminals, K and K^. The two latter can be connected at 

 will by a small short-circuit key mounted between them. This 

 key is of spring brass strip bent round at the ends (Fig. 78), 

 so that there is a slight rubbing when contact is made. When 

 required the key can be kept closed by sliding the knob H 

 under the bridge-piece B. 



Following out the connections made by the key, it will be 

 clear from the illustrations that the battery is connected to line 

 by putting the lever to one or other end of its travel, the current 

 being revsrsed in changing from one position to the other. 



In changing over, the bars touch the two intermediate 

 contacts connected to K and K;j_ ; but as these are ordinarily 

 insulated (the short-circuit key being open), the line does not 

 come in contact with earth while reversing the current. When 

 it is required to put the line to earth the lever is put to the 

 central position and the short-circuit key closed, and under 

 these conditions both poles of the battery are disconnected and 

 insulated. The usual running down of the battery, which occurs 

 when either pole has a loss on it, is therefore avoided. This 

 key, manufactured by Messrs. Elliott Bros, to the designs of 

 Mr. Eichard H. Tonking, was intended primarily to meet the 

 above requirements on board during paying out, but is also 

 adapted for other tests and uses. For instance, without altering 

 the connections a discharge test can be taken by putting the 

 lever to one side for "charge " and to the central position (with 

 the short-circuit key closed) for " discharge." For a " per- 

 centage " test (percentage loss of charge after a given period of 

 insulation) the short-circuit key is left open during the charge 

 and insulation periods, and the discharge obtained by closing it 

 immediately after the period of insulation. This effects an 

 improvement on the old method of taking this test, where two 

 keys — a battery and a discharge key — and two hands to 

 manipulate them were required. 



The key is also adapted to combine the services of the 

 reversing switch and short-circuit key ordinarily used with a gal- 



