282 Scientific Proceedings, Royal Dublin Society. 
be inserted in the coiis to increase their efficiency. This arrange- 
ment, it is obvious, is similar to those already described, but there 
can be no doubt Professor Hughes was independently led to its 
adoption in the course of his extended and admirable experi- 
ments on the microphone.* 
Another plan proposed by Hughes is the following modifica- 
tion of the double line arrangement, Fig. 6. The battery is put to 
earth in the middle, and signals are sent along the two lines, A and 
B, in opposite directions by simultaneously transmitting positive 
and negative currents along the line. The receiver R R’ is 
wound differentially, hence the currents flowing in opposite direc- 
tions conspire to actuate the instrument, but the induction cur- 
rents being in the same direction neutralize each other's effect. 
Fig. 6. 
Earth 
: Earth 
Unaware of what had been done elsewhere, I myself, during the 
past nine months, have been experimenting in spare moments in 
the endeavour to surmount induction disturbance in Telephones, 
and in a letter to The Times newspaper last autumn I mentioned 
one arrangement I had made, which seemed to promise success, 
This arrangement was an Electro-magnet, with two coils wound in 
opposite directions, which was made for me last October (1878), and 
which on an artificially disturbed line completely suppressed all 
induction. It is, however, essential for the success of the plan 
that the disturbing and the disturbed line, that is, the telegraphic 
and the telephonic line, be joined up one to each coil of the 
Electro-magnet, the former line then passing to its own instru- 
ment, the latter going to a Telephone, and thence to earth. 
This arrangement, in fact, is similar to Edison’s or Hughes’ com- 
pensating coil, but as it involved access to other lines besides the 
telephonic line, my thoughts were turned to suppress the induction 
* Telegraphic Journal, November, 1878. 
