The pressure, or voltage, of an alternating current can be stepped up or stepped down, that is, raised or lowered, by means of a transformer, which is the simplest form of induction coil. The current strength varies proportionately with the charges in pressure so that there can never be any increase in the total amount of energy but there is always a loss of energy due to heating and other causes. The moral again is that an electrically driven perpetual motion machine is a delusion and a snare. Alternating current can be changed into interrupted direct current, see Fig. 53, by an electrolytic rectifier or a mercury vapor tube.
Fig. 53. ALTERNATING CURRENT CHANGED INTO AN INTERRUPTED DIRECT CURRENT
A high tension current is an alternating current of sufficient pressure to make a jump-spark; it can be produced by a high-tension magneto, or a spark coil. An alternating current is generally considered one that changes its direction less than 100,000 times a second; when it changes its direction 100,000 times or more a second it is called an oscillating current, see Fig. 54, or a high frequency current, and this is the form of current that is used for sending out wireless waves.
Fig. 54. A PERIODIC OSCILLATING CURRENT
The only known way to set up oscillating currents of really high frequency is by discharging the stored up electricity of a condenser, or its equivalent, through a circuit of small resistance by means of a spark, or an arc. The latter sets up sustained oscillations as shown in Fig. 55. High frequency alternators (machines) have been built which generate alternating currents of over 100,000 cycles per second.
Fig. 55. A SUSTAINED OSCILLATING CURRENT
Magnetism.—A bar of steel can be made magnetic by rubbing it on a permanent steel magnet or on an electromagnet, or winding a number of turns of wire around it and passing a current through the wire.