294 proceedings: philosophical society 



While precision measurements with direct current are commonly 

 made by null methods, high-frequency measurements on the other hand 

 use the opposite critical phenomenon, a maximum rather than minimum 

 of current. Resonance or tuning is thus the basis of radio measure- 

 ments as well as of wave transmission and reception. At resonance 

 the reactances due to capacity and to inductance annul each other and 

 the current is limited by resistance only. As resistances are small in 

 comparison with reactances at radio frequencies, the current is rela- 

 tively very great at resonance, i.e., the resonance is very sharp and is 

 thus a suitable basis for measurements. 



The wavemeter is a resonance instrument, and is used to measure 

 capacity and inductance as well as wave length. The other most 

 generally useful radio instrument is the ammeter. With these two, 

 measiu'ements are made of current, resistance, power, and associated 

 quantities, in addition to the quantities above mentioned. In general 

 the best methods are those which are the least complicated. This is 

 particularly true because of the disturbing effects of small inductances 

 and capacities in lead wires and accessory apparatus. Small capacities 

 in and near the measuring circuits are especially troublesome. They 

 include the capacities of instrument cases, table tops, walls, and the 

 observer, and they cannot always be determined or eliminated. 



While resistance is of distinctly less importance in determining the 

 flow of currents at high frequencies than at low, nevertheless resistance 

 is the measure of power consumption. As it varies rapidly with ratio 

 frequencies, its measurement is very necessary. The same measure- 

 ment gives resistance and the associated quantities, sharpness of reson- 

 ance, phase difference, and decrement. 



Great advances in the precision of all these measurements have 

 recently been made possible by the introduction of electron tubes as 

 sources of current. They have the very great advantage of giving a 

 steady current, and as the current is undamped the simple sine wave 

 theory of alternating currents may be used. 



Discussion: The paper was discussed by Messrs. White, Bichowsky, 

 and SwANN. 



Mr. G. W. ViNAL then gave a paper on Some electrical properties of 

 ■silver sulphide. Silver sulphide may be prepared in the form of short 

 wires or thin strips like a metal. The wire, which must be drawn hot, 

 has been found to conduct electricity hke a metal of high specific re- 

 sistance and practically zero temperature coefficient. The strip of 

 sulphide, rolled at room temperature, has a large temperature coeffi- 

 cient and shows both metallic and electrolytic conduction at the same 

 time. It has a volt-ampere curve characteristic of a pyroelectric 

 conductor. The resistance of these strips has been examined with both 

 alternating and direct curi-ent, with the result that the alternating- 

 current resistance was nearly always found to be higher than that with 

 the direct current, and the passage of a small alternating current of a 

 frequency as low as 60 cycles increased temporarily the resistance of 

 the sulphide, while a small direct current produced the opposite effect. 



