of Edinburgh, Session 1877-78. 
709 
to determine, the crepitating sound was found not to cease even 
when b and a were brought together, but continued to be heard 
when the one wire, B, was moved upon the other A, at e. Here 
there can be no dynamic current produced in the long wire. I 
shall not detain you with my speculations on the subject ; but I 
should mention that the wire was very imperfectly insulated. 
The second experiment gives an interesting practical example of 
the interference of one pair of telephones with another pair, due to 
the splitting up of currents. The rough diagram, fig. 2, will serve 
to explain the arrangement. A and A' constitute one pair of tele- 
phones, the single connecting wire measuring 60 feet ; B and B' are 
another pair of telephones — these are connected by a copper wire 
270 feet long; for the return currents the gas pipes and earth are 
employed. The gas pipes are indicated by dotted lines ; M and M' 
are gas metres belonging to different houses ; and P and P' are the 
gas mains in different streets, and at a direct distance apart of about 
400 feet. It will be observed that the gas pipe from C to M is 
common to the two circuits. Now, it was soon noticed that when 
the battery-sounder was made use of at A, it caused a sounding not 
only in A', as intended, but also in B, sufficiently loud to be heard 
several yards off if attentively listened for. This noise was, of 
course, due to a derived current splitting off at C, and making the 
long circuit round by B, B', M', possibly to the gas main P', and 
thence to P and back by M, where it rejoins the principal current 
on its way to A. And using the battery at any one of the tele- 
phones causes more or less noise in all the four. But the small 
amount of sound produced by such straying currents is not likely 
to cause confusion. 
The Telephones used with the Battery- Sounder. — If it were 
attempted to use the sounder in combination with telephones con- 
structed in accordance with Professor Bell’s instructions — that is, 
VOL. ix. 5 A 
