1877.] Mr. W. Spottiswoode on Stratified Discharges. 547 



1. '^'^On Stratified Discharges. — III. On a Rapid Contact-Breaker^ 

 and the Phenomena of the Flow.^^ By William Spottis- 

 wooDE_, M.A., F.R.S. Received January 9, 1877. 



In a paper pubhshed in the Proceedings of the Royal Society, toI. 

 xxiii. p. 455, I have described a form of contact-breaker designed for 

 great rapidity and steadiness of action. It consisted of a steel rod 

 which vibrated under the action of an electromagnet. As regards 

 sharpness of break and steadiness of definition in the striae, this instru- 

 ment left little or nothing to be desired. But, as explained in the paper 

 above quoted, an alteration in the current not only affected the steadiness 

 directly, but also reacted on the break itself. The effects due to an 

 alteration of the current alone thereby became masked, and the study of 

 the laws relating to such changes was rendered more difficult, or altogether 

 impracticable. In order to obvdate this inconvenience I devised another 

 form of contact-breaker, in which the vibrating rod and electromagnet 

 were replaced by an arrangement purely mechanical in its action, and 

 therefore entirely under control. 



This instrument consists essentially of a wheel platinized at the edge, 

 on which a platinum spring rests. In the circumference of the wheel a 

 number (40 in the first instance) of slots were cut, and filled ^\ith ebonite 

 plugs so as to interrupt the current. The breadth of the slots was 

 about '04 inch, and that of the teeth about -5 inch. The wheel was 

 connected with suitable driving gear, so as to give from 250 to 2000 

 currents from the coil in each direction per second. A 4-inch coil 

 was found sufficient to produce the effects ; but the IB-inch coil by Apps, 

 mentioned in former communications, was preferable. With the wheel, 

 as with the electromagnetic break, a very slight strength of current was 

 required ; but, on the other hand, high tension in the primary was found 

 necessary. In many of the experiments accordingly from 10 to 20 of 

 the smallest Leclanche cells usually made were employed with the small, 

 and from 20 to 50 mth the large coil. But these were afterwards 

 replaced by a double fluid battery suggested by my assistant, Mr. P. 

 Ward, and described at the end of this communication. 



Por some time the experiments were conducted with the platinum 

 spring resting on the wheel ; and the effects were varied by altering 

 either the pressure of the spring or the velocity of the wheel ; but the 

 gradual abrasion of the platinum through friction proved to be a fruitful 

 source of irregular results. This irregularity of action, at all times difficult 

 to compensate, and sometimes insuperable, was fortunately removed by a 

 simple although delicate adjustment. It was, in fact, found that actual 

 metalhc contact between the spring and wheel was not necessary, pro- 

 vided that a layer or cushion of conducting material were interposed. 

 Such a layer was formed by a thin film of liquid dra\m out by a thread 

 leading from a reservoir and resting on the wheel. Various fluids were 



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