High Frequency Apparatus Page 131

CHAPTER XIII. HOT WIRE METER CONSTRUCTION. A Hot Wire meter is the most easily and cheaply constructed of all practical electrical measuring devices. It may be used either as an ammeter, or in series with a resistance, as a volt-meter, on direct or alternating current of any frequency, and it is the most satisfactory high frequency current measuring device known. The principle upon which a hot wire meter operates is the expansion of metals when heated. The current passes through a long, fine wire "element," which becomes longer when heated by the traversing current. The one end of this wire element is fixed stationary, while the other end is attached to a lever, which acts as an "indicator.” In this way the linear expansion of the wire causes a movement of the indicator which is proportional to the square of the current. The instrument here described will give very good results and it may be constructed at a minimum cost. Its current carrying capacity and sensitivity may be varied by changing the cross-sectional area or material of the wire used. Platinum alloy wire is generally used on commercial instruments on account of its high melting point, but for experimental work, copper wire will very often answer very well. The higher the resistance and the greater the coefficient of expansion of the wire, the more sensitive the instrument. The higher the conductivity of the wire, the greater the current it may carry for a given cross-sectional 131