If the ball is lowered until the lid is in place, the leaves take a steady deflection.
For experiments with long thin rods or wires it has an advantage over the other arrangements in that the position of the poles need not be known with great accuracy, a small upward or downward displacement having little effect upon the magnetometer deflection.
Referring to the calculations given above, this is equivalent to 1' deflection for every degree of elevation, which amount had to be given towards the higher wheel.
If a battery on the mainland is connected through a key with the shore end of the main cable, and a speaking galvanometer is in circuit with the short cable crossing the Fastnet rock, then closing or opening the battery connexion will create a deflection of the galvanometer.