688 Proceedings of the Royal Society 
the ordinary telephone in connection with Wheatstone’s bridge. 
The mathematical theory of various cases likely to prove useful is 
examined, and their application to the comparison and evaluation in 
absolute measure of electrical quantities is discussed. 
Several curious experiments with the differential telephone were 
shown to the Society. In particular it was shown that when an in- 
terrupted current was passed through one coil of the telephone so that 
the sound could he heard all over the room, by merely connecting up 
the parallel coil through a small resistance, the sound was so much 
deadened as scarcely to be audible at a distance. The deadening 
effect was shown to he much less when the resistance through 
which the idle coil was connected had considerable self-induction 
Grant’s experiment (“Phil. Mag.,” May 1880, p. 352) was also 
shown. The electrical theory of these experiments is given in the 
above paper. 
| Added, August 19.] 
Since the above abstract was written I have carried out the 
practical application of the above theory much farther. An 
instrument for giving a variable self-induction with constant 
resistance has been constructed, and promises to give very good 
results. I hope to lay a description of it before the Society next winter. 
By the kindness of Sir William Thomson and Professor Fleeming 
Jenkin, my stock of available resistance coils and capacity standards 
has lately been much increased ; and I have been able to satisfy 
myself as to the thorough practicability of all the above methods of 
electrical measurement. I reserve details in the meantime ; hut 
may mention that the differential telephone shows differences as to 
self-induction between the so-called induction-less resistance coils. 
I have measured this difference in the case of two 1000 coils in one 
of Elliot’s boxes, and find it about what might he expected from 
theory (see Maxwell, “Electricity and Magnetism,” vol. ii. p. 291). 
I believe, from preliminary experiments on capacity measurements 
with this instrument, that it will he possible by means of it to 
compare capacities of the order of a microfarad to the T q qL-_- p ar t 
(provided, of course, that capacity turns out to be definite to that 
degree of accuracy). 
Since I have had the use of standards of capacity I have been 
