PROFESSOR CHRYSTAL ON THE DIFFERENTIAL TELEPHONE. 627 
discontinuous contact and the battery were absent. The results were in all 
cases much the same, allowance, of course, being made for the difference of the 
general character of the sounds. 
It was found, under the above circumstances, that the sound was greatly 
weakened when the second coil of the differential telephone was closed. Thus, 
when the sound was so loud that it could be heard all over the room, closing 
the second coil deadened it so much that it could scarcely be heard at a 
distance at all. 
Not only is the intensity very much affected, but the quality of the sound 
is greatly altered. The high tones are much more deadened than the low, 
consequently the sound is deepened in character. 
The deadening effect was found, as might be expected, to decrease when 
the second coil was closed through a greater and greater (inductionless) 
resistance. 
Experiment 5.—The arrangements being as before, it was found that 
closing the second coil of the differential telephone through a resistance of 
9 ohms with a considerable self-induction (coefficient=°016 earth-quad. or 
thereby) has less deadening effect than closing it through 9 ohms taken from 
a resistance box, and therefore having very little self-induction. 
Experiment 6.—The effect of closing the coil through a condenser was as 
follows :—When the capacity is very small, the deadening effect is, of course, 
nil; and when it is very great, the result is the same as if the condenser were 
short circuited. It was found that in certain cases, for a given tone, closing 
the neighbouring coil through a certain capacity zncreased the sound, so that 
there was a certain capacity for which the sound was best heard, and better 
heard than when the second coil was open, and therefore idle. 
This experiment succeeds very well when a very faint sound is used, say a 
tick sent by a Bell telephone so as to be scarcely audible when the second 
coil of the receiving differential telephone is open; under these circumstances, 
the increase of the sound by closing the second coil through a condenser of 
proper capacity is very marked. 
Experiment 7.—This is a repetition of GRANT’s experiment* under more 
favourable circumstances. The sending instrument was either a microphone 
or a rasping contact. The secondary coil (resistance 44 ohms, coefficient of 
self-induction ‘2 earth-quad.) of a small induction coil, with a bundle of covered 
iron wires for a core, was inserted in the line, and the receiver was one coil of 
the differential: telephone so often alluded to above (the second coil was left 
open). This arrangement I shall call I.; when the primary (res. ‘3 ohm, 
self-ind. *004) was closed simply, we have arrangement II. ; when the primary 
was closed through a condenser, arrangement ITI. 
* Phil. Mag., May 1880. 
