XXI.—-On the Differential Telephone. By Professor CHRYSTAL. 
(Revised 20th August 1880.) 
PART IL. 
On THE GENERAL THEORY OF THE INSTRUMENT, AND ON ITS APPLICATION TO 
ELECTRICAL MEASUREMENTS. 
Some time before the telephone was invented, I had occasion to consider 
very closely the problem of the opposition offered to the passage of the electric 
current by an electrolyte, and to seek for new methods of dealing with it. It 
was not difficult to see that the telephone afforded advantages in this kind of 
electrical measurement. As far as regards the measurement of what is 
usually called Electrolytic Polarisation, these advantages are perhaps even 
greater than they might at first sight appear. In the case of what is generally 
called Electrolytic Resistance, they are, however, less than they appear. 
As an instrument for use in resistance measurements, theory and practice 
lead me to believe that the telephone is far inferior to the galvanometer. I 
have found, however, that it can be used with great advantage in the measure- 
ment of coefficients of induction and of capacities. 
The calculations in the earlier part of this paper were made more than two 
years ago; but it was not until this summer that I found leisure to bring them 
to the test of experiment. For an opportunity of so doing I am indebted to 
my colleague Professor Tart, who has put the resources of his laboratory 
at my disposal. I am also much indebted to Sir Wit11AmM TuHomson and 
Professor FLEEMING JENKIN for their kindness in lending me apparatus of 
different kinds. 
In what follows, I select from a considerable number of theoretical 
investigations in my possession mainly those cases which I have tested by 
experiment. 
The telephone indicates directly the variations of an electric current, and 
in this respect its function as a current detector is essentially different from 
all the ordinary electrical indicators. It is necessary, therefore, to work with 
"avarying current. For some purposes, as I shall show later on, it would be 
essential to have a current subject to a simple harmonic variation of known 
frequency ; but for most purposes, any kind of interrupted current will do, 
and since any periodic variation can be represented by a series of simple 
harmonic variations, I shall in all the theory that follows work with a single 
term of such a series. 
VOL. XXIX. PART II. 7M 
