THE MEASUREMENT OF CURRENT 61 



high periodicity, such as occur in telephonic work or in wireless 

 telegraphy. 



The standard resistances of the heaters are 2 and 150 ohms, 

 the latter for telephonic work. A full-scale deflection can be 

 obtained with a current of about 10 milliamp. Consequently 

 the power taken by the instrument is about 0.015 watts at full- 

 scale reading. 



To obtain a compact heater for use with currents below 20 

 milliamp. a deposit of platinum on mica is used, the platinum 

 being scraped away to form a grid. A resistance of several 

 hundred ohms may thus be obtained in a space of less than 0.2 

 sq. cm. For larger currents a wire heater is used. In either 

 case the self-induction and 

 capacity are so small as to be 

 negligible in their effects. 



High-frequency Ammeters. 13 

 By proper design the 

 thermo-ammeter may be 



adapted to the measurement FIG. 27. High-frequency ammeter for 



c -, , v e small currents, 



of large high-frequency cur- 

 rents, such as occur in radio-telegraphy. 



An instrument for small high-frequency currents is shown in 

 Fig. 27. The entire current flows through the wire AB, which 

 for currents up to 0.3 amp. may be a Eureka wire 0.05 mm. in 

 diameter. The skin effect in this wire at 1,000,000 cycles is 

 less than 0.001 per cent. For currents up to 1.2 amp. a copper 

 wire 0.08 mm. in diameter is used. In this the skin effect at 

 1,000,000 cycles is less than 0.3 per cent. The temperature of the 

 wire is determined by a thermo-electric junction. The indicator 

 may be a moving coil galvanometer connected between the 

 binding posts. 



Instruments for small currents present no difficulties. The 

 trouble arises when it is necessary to have a large carrying capac- 

 ity. In this case, two or more wires must be used in parallel 

 , and the difficulty comes from the fact that the current may not 

 divide properly between the wires. 



The heating in all the wires should contribute to the functioning 

 of the indicator. Then the errors due to the improper distribu- 

 tion of the current are much decreased, for though the total heat 



