A rheostat or impedance and reactance coil should be placed in series with the transformer to regulate the current and also to prevent arcing across the spark gap.
Reactance.—In Chapter I, the lag and lead of a circuit were explained in connection with tuning. This is a property of every alternating circuit and is brought to our notice again in the transformer which charges the condenser. The current developed by a transformer is a leading current, since the instantaneous values of the current do not correspond to the proportionate values of the voltage supplying the current. In order to force the current values of the charging current to correspond with the voltage it is necessary to produce a "lag." This is accomplished by means of an adjustable reactance in series with the primary of the transformer.
A reactance or inductance suitable for the 2-K.W. transformer may be made by building up a coil in the same manner as described under the heading of the J-K.W. transformer. The reactance will have to be somewhat larger on account of the heavier currents. The core is built up of sheet iron to measure 2 1/2 x 2 1/2 x 10 inches when completed. The coil is wound around a wooden form and is composed of about 100 turns of No. 8 B. S. gauge double cotton covered magnet wire. By varying the amount of core inserted in the hollow coil the energy may be adjusted as desired.
Fig. 51. Clapp-Eastham 1/4-K.W. Transformer.
Fig. 51 illustrates the 1/4-K.W. transformer manufactured by the Clapp-Eastham Company. The core is so constructed that a small metal tongue of soft iron projects from one side of the core towards the opposite side between the windings, but is separated from the opposite side by a small air gap. Several objects are accomplished by this tongue, which gives rise to magnetic leakage; the inductance of the primary is increased thereby to such an extent that the transformer is self-controlling, so that it may be connected directly to the source of alternating current supply of ordinary commercial frequencies and potential, and the current flowing in this circuit be regulated by varying the number of turns in the primary coil. As this magnetic leakage gives rise to a loose coupling effect, the primary and secondary circuits may be brought into resonance by placing a suitable capacity across the secondary terminals. This condition of resonance brings the power factor to a materially higher percentage. While the power factor of the open or closed core transformer is seldom above 50%, this type of transformer has a power factor of 80 to 90% when used with a suitable condenser.
Fig. 52. United Wireless Motor-Generator set for supplying Alternating Current to the Transformer.
Another point of considerable advantage is the almost entire freedom from arcing at the spark gap when this type of transformer is used. The spark gap is connected directly across the secondary terminals of the transformer and the condenser. The primary turns of the helix and the spark gap are connected in series. When the transformer is in operation, this condenser being across the secondary, the transformer is in resonance and the condenser is charged to such a point that it will jump the spark gap. At the instant that the spark passes, the secondary of the transformer is practically short circuited through the spark gap. As this circuit is now closed and the condenser out of circuit, the secondary circuit of the transformer is no longer in resonance and the energy immediately drops off, destroying at once the tendency for an arc to form. As soon as the spark has passed, the condenser of course comes in to play and the condition of resonance being reestablished the same process is repeated. The Clapp-Eastham Company have made application for a patent on any transformer employing this or any similar construction for use in charging a condenser.