_The Balance_ (Figs. 4 and 5) is made with two solenoids. S and S',
whose relative resistances is adjustable. S conveys the main current,
and is wound with thick wire having practically no resistance, and S'
is traversed by a shunt current, and is wound with fine wire having a
resistance of 600 ohms. In the axes of these two coils a small and light
iron tube (2 mm. diameter and 60 mm. length) freely moves in a vertical
line between two guides. When magnetized it has one pole in the middle
and the other at each end. The upward motion is controlled by the
spring, N T. The spring rests upon the screw, H, with which it makes
contact by platinum electrodes. This contact is broken whenever the
little iron rod strikes the spring, N T.

The positive lead from the dynamo is attached to the terminal, B, then
passes through the coil, S, to the terminal, B', whence it proceeds to
the lamp. The negative lead is attached to terminal, A, passing directly
to the other terminal, A', and thence to the lamp.

[Illustration: FIG. 4]

The shunt which passes through the fine coil, S', commences at the
point, P. The other end is fixed to the screw, H, whence it has two
paths, the one offering no resistance through the spring, T N, to the
upper negative terminal, A'; the other through the terminal, J, to the
electromagnet of the break, M, and thence to the negative terminal of
the lamp, L'.

[Illustration: FIG. 5.]

_The Cut-off_.--The last part of the apparatus (Fig. 4) to be described
is the cut-off, which is used when there are several lamps in series. It