The solenoid, S, is interposed in the general circuit, being connected on
the one hand with the line, L, and on the other with a very constant
battery of an electromotive force proportioned to the resistance of the
circuit.

Through the electrode that remains free, the battery is grounded with so
great care that no variation in resistance can be produced thereby. If the
station is near the sea, the conductor of this electrode may be run to a
copper disk, having the same surface as the one at the transmitting
station. With this description, the operation of the apparatus may be
easily understood.

At low water, the pressure of the atmosphere balances a column of mercury
rising in a glass tube to a height proportionate to such pressure. In
measure as the level of the water rises, the pressure on the mercury in the
receptacle increases, and causes the metal to rise in the tube. The higher
the level of the sea, the less becomes the sum of the resistances of the
rheostat, since the column of mercury puts in short circuit all the
divisions of the rheostat, whose contacts are comprised in the height of
the column.

From these variations in the resistance of the circuit naturally result
variations in the current from the battery, B, at the registering station.
To the variations in intensity of the current in the circuit there
correspond variations in the attraction of the solenoid for the core that
transmits these motions to the balance that carries the registering style,
which latter amplifies or reduces them.

The same transmitter suffices for various registering stations arranged in
series, as shown in Fig. 2.