LESSON TWENTY-EIGHT. THE HOT WIRE AMMETER.
The hot-wire ammeter is a device for indicating when the transmitting circuits are properly adjusted and arranged to emit the maximum amount of energy. It is placed in series in the aerial circuit so that the high frequency currents surging in the latter must pass through the meter and indicate their strength by moving the pointer a certain distance over a graduated scale.
When a current of electricity flows through a wire it develops a certain amount of heat therein. If the wire is of high resistance the heat will be great enough to cause the wire to expand. Advantage has been taken of this fact in the construction of the hot-wire ammeter. This device consists of a piece of platinum wire or a platinum alloy stretched taut between two posts. The wire is included in the aerial circuit. The platinum wire is connected to a spindle carrying a pointer in such a manner that when the heat causes the wire to expand the expansion is conveyed to the spindle and the pointer moves over the scale magnifying the motion. The greater the current flowing through the wire the greater will be the deflection of the pointer. The scale is calibrated by comparison with a standard meter to read in amperes.
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When a hot-wire ammeter is placed in circuit the latter is tuned by moving the position of the helix clips on the helix, altering the length of the spark gap and the condenser capacity until the maximum deflection is indicated. It is then removed from the circuit.