Fig. 2,590.—Four part commutator method of wave form measurement. The contact device consists of two slip rings and a four part commutator. One slip ring is connected to one terminal of the source, the other to the voltmeter, and the commutator to the condenser. By adjusting R when a known direct current pressure is impressed across the terminals, the voltmeter can be rendered direct reading.
Fig. 2,591.—Modified four part commutator method of wave form measurement (Duncan's modification). By this method one contact maker can be used for any number of waves having the same frequency. Electro-dynamometers are used and the connections are made as here shown. The moving coils are connected in series to the contact maker, and the fixed coils are connected to the various sources to be investigated, then the deflection will be steady and by calibration with direct current can be made to read directly in volts.
Fig. 2,592.—Diagram illustrating the ballistic galvanometer method of wave form measurement. The test may be made as described in the accompanying text, or in case the contact breaker is belted instead of attached rigidly to the shaft, it could be arranged to run slightly out of synchronism, then by taking readings at regular intervals, points will be obtained along the curve without moving the contact breaker. If this method be used, a non-adjustable contact breaker suffices. In arranging the belt drive so as to run slightly out of synchronism, if the pulleys be of the same size, the desired result is obtained by pasting a thin strip of paper around the face of one of the pulleys thus altering the velocity ratio of the drive slightly from unity.
Ballistic Galvanometer Method.—This method, which is due to Kubber, employs a contact breaker instead of a contact maker. The distinction between these two devices should be noted: A contact maker keeps the circuit closed during each revolution for a short interval only, whereas, a contact breaker keeps the circuit open for a short interval only.