370 Recent Advances in Telegraphy. [July, 
the bridge wire b c, in which the receiving relays are placed. 
K x and k 2 are the signalling-keys. B x and b 2 are the bat- 
teries. k 2 is an ordinary “ single-current ” key, whose 
function is simply to add or take out the battery power, b 2 . 
K x is a “ double-current ” key. When in its position of rest, 
as shown, a negative current of strength i continually flows 
to line from the — pole of B x , through the spring s, and t. 
This negative current is too weak to close the neutral 
receiving relay, and it is of the wrong kind to close the 
polarised relay. When, however, — 
Case i, this key is closed, the positive current from B x 
flows to line through the spring s x and t z . It is too weak 
to close the neutral relay, but it closes the polarised 
relay. 
In Case 2, when k 2 is closed and K x is open, the -f pole of 
b 2 is to earth through t 2 and springs s 2 and s x , while the 
— pole of B x is to line through s and t. Thus both 
batteries adt conjointly, and a negative current of triple 
strength flows through the line. This has the effedt of 
closing the neutral relay and leaving the polarised relay 
open. 
In Case 3, similarly, it will be found that a positive cur- 
rent of triple strength enters the line. This closes both 
relays. 
In Case 4, when both keys are open, as shown, the — pole 
of B x is to line, and a negative current of strength 1 
flows. It has the effedt of keeping both relays open. 
Fig. 10 represents the arrangement of receiving relays 
with their local circuits. r x is the polarised and r 2 the 
neutral relay. The local circuit of r x is made up of a local 
“ sounder ” and adtuating battery ; that of r 2 is made up of 
a local relay battery and “sounder.” The addition of this 
local relay is to overcome the defedt experienced in the early 
systems of the breaking-up of signals in the neutral relay 
due to sudden reversals of current in the line. A little con- 
sideration will show that the current may be reversed in the 
line by the key k x while a signal is being made on k 2 . This 
reversal of polarity in the relay involves an intermediate 
moment of no polarity, during which the tongue t 2 will fall 
away from its contadt. This causes a break in the signal 
made by the local sounder. The intervention of a local 
relay, however, was found to delay the adtion of the sounder, 
so that this interval of falling off could be bridged over, so 
to speak. On long lines the static indudtion, by delaying 
the change of polarity in the relay, extends this interval so 
