254 
THE TELEGRAPH. 
remain at rest. But the current of water passing through l 
can cause the turbine b to revolve, unless another current of 
equal pressure enters B, in which case all the water passing 
into A must find exit through A L, and the turbine will then 
turn. We see, therefore, that by admitting and stopping the 
water at b we can control the movements of the turbine A ; and 
by like means b can at the same time be controlled at A. 
The duplex system that we have just described is called the 
“ differential method ” because the coils of the apparatus are 
wound differentially, that is to say, in opposed directions, so 
A L 
that combinations may'be obtained on each coil separately, or 
in the two coils together, or by opposing their effects on the 
needle. Apparatus of this kind are much used in testing the 
electrical resistances of the line and batteries, but it is 
unnecessary to here give a detailed description of them. 
Wheatstone’s bridge supplies another practical mode of 
making the telegraph duplex, by placing an ordinary receiving 
apparatus in the bridge of the balance known by that name. 
It is necessary only to put the two ends of the common needle 
apparatus in contact with the real and the artificial line 
respectively (fig. 180). The branches in which the current is 
divided are here quite separate from the receiving apparatus. 
The rest of the connections are precisely the same as in the 
former case. 
