
Hot-Wire Ammeter for measuring Small Currents. 595 
place and then at another nearer place closer by a distance 
/ ems., then the difference of the inductances measured in 
the two cases has a value L such that; 
L=214 4606 logy - a ems. 
This formula is easily derived from one given by Maxwell. 
It is a simple matter to obtain in this manner an inductance 
having a value say of 30,000 cms., and by its aid to test 
methods of measurement. 
By the use of a long solenoid having an inductance pre- 
determined by the rule given above, the method can be used 
for the determination of small capacities of the order of a 
thousandth of a microfarad. It is to be hoped, therefore, that 
in future those who describe experiments or appliances such 
as wireless telegraphy arrangements in which such small 
capacities or inductances are used, will cease from the practice 
of speaking of jars with so many “square inches or square 
centimetres of coated surface,” and take the slight trouble to 
measure and record the capacity and inductances, and in this 
way afford the means of testing theories of the operation of 
the appliances. 
Tt can hardly be said that the practical problem of measuring 
with great accuracy very small inductances of the order of 
1 microhenry or less has been satisfactorily solved. 
Probably in the case of inductances of very low resistance 
the best method to adopt would be to measure the fall of 
potential down the conductor first, with a continuous current, 
and then with a high-frequency sine form alternating current. 
Professor W. Stroud and Mr. J. H. Oates recently described 
a bridge method employing alternating currents, which they 
stated could be employed for the measurement of very small 
inductances *. 


LXV. Ona Hot-Wire Ammeter for the Measurement of very 
small Alternating Currents. By J. A. Furmine, D.Sc., 
LRS., Professor of Electrical Engineering in University 
College, Londont. 
HERE are many occasions on which it becomes necessary 
to measure a small alternating current of the order of 
one-hundredth of an ampere. 
In taking the magnetizing currents of small transformers 
* See Prof. W. Stroud and Mr. J. H. Oates on the ‘“ Application of 
Alternating Currents to the Calibration of Capacities and Inductances,” 
Phil. Mag. ser. 6, vol. vi. p. 707 (1903). 
Tt Communicated by the Physical Society : read March 25, 1904. 
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