Separator and Shunt for Currents of High Tension. 353 



The sulphur-cell might also perhaps be used as a thermo- 

 scope, being simpler and more easy to construct than an 

 ordinary thermopile. 



It has recently been proposed to use mixtures of sulphur 

 and graphite for making cheap resistance-boxes. It is needless 

 to point out that the great sensitiveness of such compounds to 

 small changes of temperature renders them very unsuitable 

 for such a purpose. 



XLIII. On a Separator and a Shunt for alternate Currents of 

 High Tension. By William Spottiswoode, M.A., LL.D., 

 D.C.L., President of the Royal Society*. 

 [Plate IX.] 



IN a former communication (Phil. Mag. 1879, vol. viii. 

 p. 390; see also Proceedings of the Royal Institution, 

 March 1882) I described a mode of exciting an induction- 

 coil by means of an alternate magneto-electric machine. The 

 machine then, and at present, used for the purpose was one 

 by De Meritens. It answers its purpose perfectly; but it is 

 not improbable that other alternate machines might also suc- 

 ceed. The method offers various advantages, several of which 

 are described in the second paper to which reference is made 

 above ; but it also presents one considerable disadvantage — 

 namely, that the currents produced in the secondary circuit 

 are alternate in direction as well as in time. For some pur- 

 poses this is of no consequence ; but for others, such as expe- 

 riments with vacuum-tubes, it gives rise to inconvenience by 

 the production of two opposite discharges of the same strength 

 through the tube. As these succeed one another with great 

 rapidity, both sets are visible together, and by their presence 

 they mask the very effects which are the subject of study. 



With a view to remedy this defect many contrivances were 

 devised ; and among them it is difficult to decide how much 

 is due to my assistant, Mr. P. Ward, and how much to myself. 

 But it is right to state that much credit is due to him for many 

 valuable suggestions in principle, as well as for the instru- 

 mental construction. 



The first object proposed was to get rid of one set of dis- 

 charges, either by preventing its passage by means of resist- 

 ances, or by diverting it from the tube into a different channel. 

 The main principle of which we availed ourselves for this pur- 

 pose was found in the known property of vacuum-tubes, that 

 a discharge is more easily effected when the negative terminal 



* Communicated by the Author. 



