500 Wadswo?i/i — Interrupter for Large Induction Coils. 



stock of them being kept constantly on hand. With a mean* 

 current (as indicated by a direct reading Weston ammeter 

 (W, tig. 1) placed in the primary circuit) of 5 to 6 amperes, 

 a brush usually lasts four or five or sometimes ten hours with- 

 out being replaced. No other attention is required except to 

 oil the bearings of the motor occasionally. 



The only noise is that caused by the whirr of the revolving 

 parts and the snap of the spark, and this is almost completely 

 deadened by supporting the motor on rubber blocks and enclos- 

 ing the whole arrangement in a small wooden box (indicated 

 by the dotted lines in fig. 1). 



The circumferential speed is so high, nearly two thousand 

 feet per sec. that the break is extremely rapid and sharp. In 

 the first instrument constructed the axis was vertical and the 

 wheel was made to revolve under alcohol, but a short trial 

 showed that this precaution was unnecessary. 



If very heavy currents (15 or 20 amperes or higher) were 

 continually used it would probably be found advantageous to 

 place the contact-breaking brush between the poles of a power- 

 ful electromagnet (excited by the motor circuit). This would 

 blow out the arc formed on breaking the 

 circuit and thus greatly diminish the 

 rate of burning away of the brush (see 

 fig. '6). For ordinary work however 

 neither this nor the use of a condenser 

 Fip 3 is necessary although I have shown the 

 latter connected to the interrupter in 

 fig. 1. 



An alternating current dynamo properly wound is in many 

 respects the most desirable means of exciting large coils, but 

 on account of the expense of the dynamo itself and the cost of 

 running it it is not often available. Even when it is there is 

 one respect in which the action is inferior to that of an inter- 

 rupter, viz : in the rapidity of the fall of electromotive force 

 in the primary circuit. In this respect as well as in that of 

 simplicity and cheapness the rotating wheel interrupter is 

 superior to both the dynamo, and to all other forms of inter- 

 rupters, and I do not think that any one who once gives it a 

 fair trial will ever again use either a spring or mercury " break," 

 for any but the very lightest work. 



Addendum. — When this form of interrupter was first used 

 it was noted that the spectrum of the spark at the " break " 

 was very brilliant and pure and the possibility of using this 

 spark as a source of radiation instead of an electric arc, sug- 

 gested itself although not in any very definite form. This 



* The maximum strength of current at the moment of breaking the circuit is, 

 with the speed usually adopted, about twice the mean current as indicated above. 



