232 Mr. Albert Griffiths : Some 



nating current, and I attributed the imperfections, perhaps 

 erroneously, to slight irregularities in the make and break. 



The interruptions in the primary circuit were caused by a 

 platinum point leaving a mercury surface which was covered 

 with methylated spirit. The spark seemed to me to be 

 an erratic factor, and I endeavoured to get rid of it by 

 employing a resistance as a shunt across the spark-gap, by 

 employing a condenser as a shunt, and in particular by a 

 method of Differential Winding, which was only a partial 

 success, described by S. P. Thompson in * The Electro- 

 magnet and Electromagnet Mechanism.' 



The method of Differential Winding diminishes the spark 

 on break, but certainly does not eliminate it ; as the action 

 of the arrangement is of some interest, 1 have given a full 

 explanation of its construction and behaviour in Addendum I. 



The device for preventing sparking which acted the best, 

 and which I think is a new one, consists in the use of electro- 

 lytic cells or small batteries placed as a shunt across the 

 spark-gap. 



I first used electrolytic cells, the platinum plates of which 

 were about 1 in. by | in., and dipped in dilute or strong sul- 

 phuric acid. 



The explanation is that the cells polarize, and on making 

 the gap an E.M.F. is introduced opposed to the E.M.F. of 

 the battery, so that the current rapidly diminishes, decom- 

 posing the liquid and doing chemical work. 



I may mention, incidentally, that the rapid make and break 

 of current through the electrolytic cells disintegrated the sur- 

 face of either the positive or negative plates (I did not observe 

 which) and brought platinum into suspension, in a very finely 

 divided state, throughout the liquid. 



I tried plates of gas-charcoal in dilute H 2 S0 4 ; these did 

 not act so well as the platinum. 



I tried lead plates in dilate H 2 S0 4 ; these acted quite as 

 well as the platinum plates, and are of course cheap. 



When the above-mentioned device was employed there was 

 no trouble with the mercury ; in fact the little sparking there 

 was seemed to purify rather than contaminate the surface. 



Having now had some experience with alternating cur- 

 rents, I was recommended to examine some of Lenard's 

 experiments* on the electrical resistance of bismuth in a 

 magnetic field with constant and alternating currents. 



The telephones and bridge employed were those which are 

 used for the determination of electrolytic resistances by Kohl- 

 rausclr's method ; the bridge-wire is wound on a cylinder. 



* Wied. Ann. xxxix. p. 619 (1890). 



