Wireless Telegraphy and Telephony

74 WIRELESS TELEGRAPHY The accompanying diagram serves to illustrate the prin- ciple of the “hot-wire” meter. A piece of platinum wire is stretched tightly between two rigidly fixed posts. A thread leads from the center of the “hot wire” to a small spindle around which it passes once or twice. The spindle is also connected to a spring which exerts a continual tendency to turn the spindle but is prevented from so doing by the Fig. 90. Diagram showing loose coupled helix in circuit. thread attaching to the hot wire. Any tendency on the part of the string to slacken a little, however, will imme- diately permit the spring to turn the spindle. When a high frequency current is passed through the platinum wire it becomes heated and expands. The expansion of the wire allows the thread to slacken slightly with the immediate result that the spindle turns. The spindle carries a pointer at the upper end which shows the amount of turning. It is therefore easy to tell the comparative strength of current flowing accordingly as the deflection is great or small. The meter is placed in series with the aerial and when the high frequency currents pass through it they heat and