MEASURING, TESTING, AND INSTRUMENTSMEASURING, TESTING, AND INSTRUMENTS160. The magneto test set is one of the most valuable testing instruments for practical people because of its simplicity and the fact that it is always ready for service. Figure 1.58 shows the circuit and Fig. 1.59 a perspective view of a testing magneto. The apparatus consists of a small hand-operated ac generator in series with a polarized electric bell. Alternating current will ring bells of this type. If the external circuit connected to the terminals of the magneto is closed and the crank of the generator is turned, current will flow and the bell will ring.