dellinger: high frequency ammeters 413 



dynamometer, conditions are favorable (impedance large and 

 capacity large) for part of the current to flow thru the dielectric 

 instead of the wire, in amount varying with the frequency. The 

 superiority of the hot-wire ammeter and the modifications there- 

 of, in high frequency work, is due to the simplicity of form which 

 the portion of the circuit within the instrument may have, per- 

 mitting a minimum of self-inductance and capacity, A single 

 straight wire of very small diameter is the only form of ammeter 

 circuit which can be taken as a 'priori reliable at all frequencies. 

 When the instrument is required to carry relatively large cur- 

 rents, such a wire is not sufficient, and more than one elementary 

 path must be provided for the current. The distribution of cur- 

 rent among these paths is determined solely by the resistances, 

 at low frequencies, while at high frequencies the inductances pre- 

 dominate. Consequently the current distribution and the read- 

 ings of the instrument are likely to change as the frequency is 

 varied. 



There are three general types of ammeters in use for large cur- 

 rents of high frequency: (a) that employing wires in parallel, 

 (b) the so-called unshunted ammeter in which a single wire has 

 different portions of its length in parallel, and (c) the ammeter 

 employing thin metal strips. Both experimental and commer- 

 cial instruments of all these types were investigated, experimen- 

 tally and theoretically. All types were found to be subject to 

 errors at radiotelegraphic frequencies. This fact is of great 

 moment, for the ammeter is the cardinal instrument in high fre- 

 quency work. It is of value in measurements of resistance and 

 power, as well as of current. In some of the ammeters investi- 

 gated the readings were found to increase with increase of fre- 

 quency, and in others to decrease. These changes are entirely 

 independent of the thermometric method used to measure heat 

 production. The thermometric device may depend on expan- 

 sion, calorimetry, electric resistance, or thermoelectric effect. 



The observations were made by passing high-frequency and 

 low-frequency current successively thru the instruments. An 

 instrument under test was always in series with an instrument 

 which could be taken as standard, and the two were observed 



