is brought into play by the switch, C D, which can be placed at E or D.
When it is at E, the negative terminal, A, is in communication with
the positive terminal, B, through the resistance, R, which equals the
resistance of the lamp, which is, therefore, out of circuit. When it is
at D the cut-off acts automatically to do the same thing when required.
This is done by a solenoid, V, which has two coils, the one of thick
wire offering no resistance, and the other of 2,000 ohms resistance. The
fine wire connects the terminals, A' and B. The solenoid has a movable
soft iron core suspended by the spring, U. It has a cross-piece of iron
which can dip into two mercury cups, G and K, when the core is sucked
into the solenoid. When this is the case, which happens when any
accident occurs to the lamp, the terminal, A, is placed in connection
with the terminal, B, through the thick wire of V and the resistance, R,
in the same way as it was done by the switch, C D.

_Electrical Arrangement_.--The mode in which several lamps are connected
up in series is shown by Fig. 6. M is the dynamo machine. The + lead is
connected to B1 of the balance it then passes to the lamp, L, returning
to the balance, and then proceeds to each other lamp, returning finally
to the negative pole of the machine. When the current enters the balance
it passes through the coil, S, magnetizing the iron core and drawing
it downward (Fig. 4). It then passes to the lamp, L L', through the
carbons, then returns to the balance, and proceeds back to the negative
terminal of the machine. A small portion of the current is shunted off
at the point, P, passing through the coil, S', through the contact
spring, T N, to the terminal, A', and drawing the iron core in
opposition to S. The carbons are in contact, but in passing through
the lamp the current magnetizes the electromagnet, M (Fig. 2), which
attracts the armature, A B, that bites and lifts up the rod, T, with the
upper carbon, a definite and fixed distance that is easily regulated