708 Prof. Stroud and Mr. Oiite.< on the Apiyiication of 



measurements. "Within the last few months, liowever, we 

 have succeeded in increasing the sensitiveness of our appa- 

 ratus to such an extent that we can detect a capacity of say 

 one-third of a micro-microfarad, and we can measure with 

 certainty a small inductance to at least two microhenries,, i. e. 

 one five-hundredth of a millihenry. 



In connexion with this question of the detection of a 

 capacity amounting to a fraction of a M.M.F. (1 M.M.F.= 

 1 micro-microfarad = 0'9 electrostatic unit), it should be 

 remembered that J. J. Thomson and Searle* and E. Kosaf 

 succeeded in detecting very small capacities in thefr respec- 

 tive determinations of ^'r." These experimenters were, 

 however, using very sensitive high resistance galvanometers. 



Our solution of the problem is in some respects simpler 

 than that of Prof. Fleming. As to sensitiveness our appa- 

 ratus is probably not quite so sensitive for the measurement 

 of very small capacities, whereas for the measurement of 

 small inductances it is, we think, superior. On the other 

 hand, our apparatus is not so suituable, in its present form 

 at all events^ for effecting the accurate measurement of 



The essential feature of our method consists in the employ- 

 ment of what is substantially an electrodynamometer with 

 laminated iron cores, or it may be brief!} described as a 

 movable-coil D^Arsonval galvanometer, in which the per- 

 manent magnet is replaced by an electromagnet, with a 

 laminated iron core, actuated by a 100- volt alternating 

 current. The solid iron core inside the moving coil of the 

 ordinary D^Arsonval is in our instrument also replaced by a 

 laminated one. 



DonleJ has used a Bellati-Giltay electrodj-namometer for 

 the measurement of specific inductive capacities in an entirely 

 different way. The oscillating current from an induction- 

 coil (without iron core) was passed through a condenser 

 (with adjustable plates) joined in series with the electro- 

 dynamometer and the deflexion was observed. On inserting 

 a parallel slab of the dielectric between the plates and re- 

 adjusting their positions this deflexion was approximately 

 reproduced. 



Our instrument is used just like an electrodynamometer 

 wdien it is arranged for measuring the conductivity of elec- 

 trolytes, i. e. the field-magnet is placed across the mains, and 

 the highly insulated movable coil is used to replace the gal- 

 vanometer in the bridge. We have found this piece of 



* Phil. Trans, vol. clxxxi. A (1890). f Phil. Mag. Oct.' 1889. 



X Wied. Ann, xl. p. 307 (1890), 



