186 Messrs. Edser and Stansneld on a Portable 



cannot suppose that imperceptible traces of grease, inera- 

 dicable as they may be, would produce complete and delicate 

 outlines. The cleaning off of impressions may at first seem 

 to indicate a deposit ; but this renewal of the surface might 

 rather be like smoothing out an indented tin-foil surface : 

 such a view might explain the case where a blank over- 

 electrified disk is developed into fine detail. The electrified 

 figures seem to point to a bombardment, which produces a 

 molecular change, the intensity of electricity bringing about 

 quickly what may also be done by slow persistent action of 

 mechanical pressure. At present it seems as if most of the 

 phenomena cannot be drawn out from the unknown region 

 of molecular agency. 



While experimenting I was not within reach of references 

 to former researches, but I have since done my best to find 

 them out, and to indicate all I have learnt in the body of my 

 paper. 



Poggendorff, vol. lvii. p. 492 ; translated in Archives de 



I'Electricite, 1842, p. 647. 

 Riess' Electrische Hauchfiguren in Repertorium der Physik; 



translated in Archives de I'Electricite, 1842, p. 591. 

 Riess' Die Lelire von der Reibiuigs Elect ricitdt, vol. ii. 



pp. 221-224. 

 Mascart, Electricite Statique, vol. ii. p. 177. 

 Taylor's Scientific Memoirs, vol. iii. 



XXI. A Portable Instrument for Measuring Magnetic Fields. 

 With some Observations on the Strength of the Stray Fields of 

 Dynamos. By Edwin Edser and Herbert Stansfield*. 



[Plates III. & IV.] 



THIS instrument was constructed for the purpose of giving 

 direct readings for the strength of magnetic fields, such 

 as are found in the neighbourhood of Dynamos ; thus avoid- 

 ing the inconveniences attending the Ballistic method. Porta- 

 bility, a considerable range, and a fair degree of accuracy 

 were the qualities sought. The instrument, as now con- 

 structed, whilst satisfying the first of these conditions, may 

 be used to measure any field from 1 line per centimetre up- 

 wards, with an error of only about 2 per cent. ; the only 

 accessories required being a dry cell and a resistance-box. 



* Communicated by the Physical Society : read May 13, 1892. 



