Measurement of Large and Small Alternating Currents. 91 
studies of solar radiation is of increasing interest, in view of 
their possible aid in forecasting terrestri al Shae changes, 
conceivably due to solar ones. 
All the preceding observations, while under my general 
direction, have been conducted in detail by Mr. ‘Abbot, to 
whom my thanks are especially due. 
Washington, ae 15th, 1904. 
IX. oe ee, Pe the Measurement of Large and Small 
Alternating Currents. By W. Duppeui, Wh.Sc.* 
[Plate ITT. ] 
OR the measurement of alternating currents over the 
range from 0°1 to 100 amperes, there are a large number 
of commercial instruments available which will give fairly 
accurate results and which are quite easy to use. As soon 
as one requires to measure a current much outside this range, 
say 735 ampere or 1000 amperes, then more or less serious 
difficulties are encountered, even if a moderate accuracy of 
say 2 or 3 per cent. is required. I propose to describe in this 
paper some instruments which I have devised, by means of 
which very small alternating currents and voltages ean be 
measured, and which are alse suitable when shunted for very 
large currents. 
In order that an instrument shall read R.M.S. or effective 
amperes, it is necessary that its deflexion shall be determined 
by the mean squared value of the current flowing. This 
leads to a difficulty when it is required to construct instru- 
ments for very small currents, which I will illustrate by an 
example. 
Suppose an instrument gives a certain deflexion for 1 ampere 
and that it is required to alter it to give the same deflexion 
for a milliampere, then the available deflecting force, which 
depends on the square of the current, will be reduced in the 
ratio of one million to one. This is the reason why so little 
advance has been made in the measurement of very small 
alternating currents compared to that made in the measurement 
of direct currents. 
The means at our disposal to measure alternating currents 
may be roughly divided under three heads according to which 
of the effects produced by the current is taken as a measure 
of ‘it. 
(1) Electromagnetic Instruments——These may be further 
subdivided into two classes, namely: (a) Pure-air core-dyna- 
mometers, in which the current to be measured flowing 
* Communicated by the Physical Society : read May 6, 1904. 
