708 
Proceedings of the Royal Society 
the telephones be of suitable construction, will suffice to elicit 
sufficient noise at the distant station to call the attention of any one 
in the same or even an adjoining apartment ; the noise given out 
by the distant telephone will not be altogether due to this diverted 
current, but will owe part of its volume of sound to telephonic 
sympathy with the great agitation produced in the one near the 
operator. 
Before entering into any details as to the construction of tele- 
phones best suited for this mode of sounding a “ call,” and for short 
distances, permit me to describe two experiments upon the detec- 
tion of derived currents by means of the telephone. 
Fig. 
1 . 
Fig. 1 represents a wire of about 320 feet in length, bent so as to 
bring the middle of its length near to the telephone T, to which its 
extremities are attached ; G is a battery of one cell, and A and B 
the wires from its electrodes ; one of these, A, is fixed to the long 
wire at a ; the other, B, is movable at b , the point b being taken at 
different distances from a. When the length of a-b is taken equal 
to from 3 to 4 feet, the noise emitted by the telephone might 
be sufficient to act as a call, although only about 1 per cent, of the 
current from the battery can then pass through the telephone ; 
when the length of a-b is reduced to 1 foot, the sound from the 
telephone may yet be heard several feet away, and as the distance 
of a-b is decreased, the telephone must be brought nearer and 
nearer to the ear in order to hear the crepitating sound ; with a-b 
equal to 3 inches, it may be heard 3 inches off ; and when the 
telephone is held against the ear the distance of a-b may usually be 
reduced to less than a quarter of an inch before the sound is lost. 
Now, when the length of a-b is one quarter of an inch, the strength 
of the diverted current passing through the telephone used would 
only amount to the one twenty-thousandth (- 2 - ^ w ) of that circulat- 
ing through the battery. But I have now to note a very puzzling 
circumstance. Sometimes-, under conditions I have not been able 
