182 Trayisactions. — Miscellaneous. 



Currents" (Proceedings, Eoyal Society, 17th Januarj-, 1889,. 

 vol. xlv.), investigates mathematically the screening produced 

 by a conducting-plate, and shov/s that it is proportional to the 

 thickness, and that if plates of different metals produce the 

 same screening their thicknesses will be proportional to their 

 specific resistances. 



The screening mentioned by Professor Thomson is mea- 

 sured by the decrease of electro-motive force, while in these 

 experiments the screening is measured by the decrease of 

 magnetic force ; and it is possible to have very considerable 

 screening of electro-motive force and at the same time very 

 little screening of magnetic force, for the magnetic field may 

 be large though its rate of change is small. Professor Thom- 

 son (see paper mentioned above) found that a thickness of 

 __!__ of a centimetre of Dutch metal screened off all electro- 

 motive force, and the thinnest films of metal he could obtain 

 screened ofi" all effect. 



Art. XX. — Magnetic Viscosity. 



By E. EuTHEKFORD, M.A., B.Sc, 1851 Exhibition Science 



Scholar. 



{Read before the Philosophical Institute of Canterbury, 4th September, 



1895.] 



This research was undertaken to see if steel or soft iron 

 exhibited any appreciable magnetic viscosity when under the 

 influence of very rapidly changing fields. Ewing had shown 

 that there was a slow creeping-up of the magnetization for 

 some seconds after the magnetizing force had been applied ; 

 but considerable difference of opinion has been expressed as to 

 whether the area of the hysteresis curve would be less for a 

 slow cycle than for a very rapid cycle of less than j-^qj^ of a 

 second. 



I had already designed the apparatus and the method of 

 reducing the experiments before a copy of the Proceedings of 

 the Eoyal Society, 20th April, 1893, reached New Zealand. I 

 there found an account of experiments by Messrs. Hopkinson, 

 Wilson, and Lydall, which in a great measure anticipated 

 what I had intended doing. Later, wdien I received a copy 

 of Gray's Absolute Measurements, I found an account of 

 recent researches on the same subject (vol. ii., 753-758). 



Messrs. Evershed and Vigneroles had shown that there 

 was very little difference between the energy lost in magnetic 



