One of the instruments used to measure electricity is called a galvanometer. It depends for its usefulness on the principle of magnetism. There are many varieties of this device. The D'Arsonval galvanometer (Fig. 79) consists of a horseshoe magnet placed vertically. Between the poles of the magnet there is an iron cylinder; above the cylinder is suspended a fine wire wound on a thin copper frame so that it will swing freely between the cylinder and the magnet poles. When the current is sent through the coil it becomes a magnet which is acted upon by the horseshoe magnet which causes it to be deflected. The deflection is measured on a scale which gives the measurement of electricity.

Fig. 77 - Cells Arranged in Series.

Fig. 78 - Cells Arranged in Parallel.