[Fig. 2,592], shows the necessary apparatus and connections for applying the ballistic galvanometer method. The contact breaker consists of a commutator having an ebonite or insulating segment and two brushes.
In operation the contact breaker keeps the circuit closed during all of each revolution, except the brief interval in which the brushes pass over the ebonite segment.
The contact breaker is adjustable and has a scale enabling its various positions of adjustment to be noted.
Ques. Describe the test.
Ans. The contact breaker is placed in successive positions and galvanometer readings taken, the switch being turned to F, [fig. 2,592], in measuring the current wave, and to G in measuring the pressure wave. The results thus obtained are plotted giving respectively current and pressure waves.
Figs.. 2,593 and 2,594.—Two curves representing pressure and current respectively of a rotary converter. Fig. 2,593, pressure wave V, fig. 2,594 current wave C. These waves were obtained from a converter which was being driven by an alternator by means of an independent motor. The rotary converter was supplying idle current to some unloaded transformers and the ripples clearly visible in the pressure wave V, correspond to the number of teeth in the armature of the rotary converter.
Ques. How is the apparatus calibrated?
Ans. By sending a constant current of known value through the resistance R.