in an iron core introduced into a coil is to distort the current wave by adding harmonics so that the ascending and descending portions may not be symmetrical. A peaked wave has a large maximum as compared with its virtual value. A peaked wave is produced by a machine with concentrated winding.

Joubert's Method.—The apparatus required for determining the wave form by this step by step method, consists of a galvanometer, condenser, two, two way switches, resistance and adjustable contact maker, as shown in fig. 2,589.

Fig. 2,589.—Diagram illustrating Joubert's step by step method of wave form measurement.

Ques. Describe the method of making the test.

Ans. For current wave measurement switch No. 1 is placed on contact F, and for pressure wave measurement, on contact G, switch No. 2 is now turned to M and the drop across the resistance (assuming switch No. 1 to be turned to contact F) measured by charging the condenser, and then discharging it through the galvanometer by turning the switch to S. This is repeated for a number of positions of the contact maker, noting each time the galvanometer reading and position of the contact maker. By plotting the positions of contact maker as abscissæ, and the galvanometer readings as ordinates, the curve drawn through them will represent the wave form.

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.

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.

Zero Method.—In electrical measurements, a zero method is one in which the arrangement of the testing devices is such that the value of the quantity being measured is shown when the galvanometer needle points to zero.

In the zero method either a contact maker or contact breaker may be used in connection with a galvanometer and slide wire bridge, as shown in