248 Prof. C. Y. Boys on Photographs of Rapidly Moving 



and upwards, the b and c curves for the same linear current 

 of 2 amperes tend to terminal coincidence, hut diverge at the 

 intermediate stages. It is proposed to study more fully these 

 relations, and to extend the investigations to nickel and cobalt. 

 In the course of these experiments an effect was noticed 

 which demonstrates the extraordinary complexity of magnetic 

 distribution in a magnetized wire. If, before the wire has 

 been magnetized at all, a current is passed along it no appre- 

 ciable longitudinal polarity is produced, as measured by a 

 magnetometer-needle placed as in the above experiments. 

 Suppose, however, that the wire has been pretty strongly 

 magnetized ; and that, in the manner discovered by Auerbach, 

 and used almost universally now, the wire is demagnetized 

 by reversals of gradually diminishing magnetizing currents 

 until the magnetometer-needle stands almost exactly at zero. 

 It is of course generally recognized that the wire so demag- 

 netized cannot be regarded as being in anything like the same 

 condition as it was in its originally unmagnetized state, 

 although it appears to have lost polarity. That this view is 

 correct may be at once demonstrated by simply passing a 

 pretty strong current along the wire, when a very pronounced 

 polarity will be evidenced by a comparatively large de- 

 flexion of the magnetometer-needle. Reversing the current 

 will reverse this polarity. Heating to a red heat can alone 

 truly demagnetize an iron wire. 



Imperial University, Tokyo, 

 May 30, 1890. 



XXX. Notes on Photographs of Rapidly Moving Objects, 

 and on the Oscillating Electric Spark. By C. V. Boys, 

 A.R.S.M., F.R.S., Assistant Professor of Physics at the 

 Normal School of Science and the Royal School of Mines, 

 South Kensington *. 



[Plate VI.] 



1. X HAD occasion last Christmas to show to a juvenile 

 JL audience a modification by the late Dr. Guthrie of one 

 of Plateau's experiments, which beautifully illustrates the effect 

 of the surface-tension of a liquid during the growth and for- 

 mation of a drop. Under ordinary circumstances a drop of 

 water is so small that the formation of the neck and separa- 

 tion of the drop are too rapid for it to be possible to follow 

 the process by the eye. If the drops were larger the process 

 would be slower, but a larger drop, owing to the weight of 

 water, cannot be formed in air. Dr. Guthrie caused the drop 

 * Communicated by the Physical Society : read May 2, 1890. 



