of Edinburgh, Session 1879-80. 
685 
The exceptional behaviour of iron in the case of thick wires 
(in very thin wires its superiority is doubtful) is not surprising. 
Professor Tait has suggested a farther anomaly, viz., that at a 
very high temperature iron may be incapable of producing these 
sounds altogether. 
I am at present in possession of some very interesting results 
bearing on this point, which 1 propose to lay before the Society at 
some early meeting. 
It happens, very curiously, that Mr Preece, apparently about the 
same time as myself, was led to devise an instrument practically 
identical with the one I exhibited to the Society. An account of it 
appeared in “JSTature,” vol. xxii. p. 138; being an abstract of the 
Proceedings of the Royal Society of London, on May 27th.] 
BUSINESS. 
The following candidates were balloted for, and declared duly 
elected Fellows of the Society: — Mr W. F. King, Professor Mac- 
Gregor, Halifax, N.S., Mr Patrick Geddes, and Dr W. Robert 
Smith. 
Monday, 21 st June 1880. 
Pbofessor MACLAGAN, Vice-President, in the Chair. 
The following Communications were read : — 
1. On the Differential Telephone, and on the application of 
the Telephone generally to Electrical Measurement. By 
Professor Chrystal. 
The plans and calculations in this paper are now more than two 
years old, but the author has only lately, by the kindness of Pro- 
fessor Tait, found opportunity to carry them out in practice. 
A discussion is given of the different methods of applying the 
telephone to accurate measurement, and mention is made of the 
points which the author thinks have been missed by most of those 
who have worked in this way hitherto. 
A common principle runs through all telephonic null methods, 
viz., that the balance may be dependent on the frequency of the 
interrupted current or may be independent of it. In the latter case 
