High Frequency Apparatus Page 132

132 HIGH FREQUENCY APPARATUS سلسل Shunt connected here Fig 1 Fig. 1.-Meter with cover removed area. The temperature of the wire should not go too high, because the higher the temperature of the wire above that of the surrounding atmosphere, the greater the loss of heat by radiation, and hence the greater the inaccuracy of the instrument. Therefore, the material and cross-sectional area of the wire must be determined by the current and use for which the instrument is intended. It is best to experiment with several wires to find which is best adapted to the purpose. The hot wire meter is absolutely "dead-beat," i. e., the pointer does not fluctuate but comes to an absolute rest as soon as it reaches the extent of its travel. To offset this pronounced advantage, the instrument is slow to record. The hot wire element takes a certain amount of time to assume its final temperature and degree of expansion and the pointer moves slowly over the scale. If a very fine wire is used for the element, the action is hastened and the inability of the fine wire to carry any great amount of current