Measurement of large and small Alternating Currents. 271 



so as to be continuous with the flow back along the other 

 face of the wave. Unless these longitudinal flows are taken 

 into consideration the whole energy of the wave is not 

 accounted for. If the rate of variation of intensity over 

 the surface be comparable with a wave-length, as in the case 

 of fine gratings, the longitudinal component is a large part of 

 the phenomenon, and, in fact, represents a large part of the 

 energy in this case transmitted to the secondary image. This 

 is all quite obvious in the case of gratings from the ordinary 

 theory, for the equations given as a solution of this case 

 represent a series of waves being transmitted in different 

 directions from the grating corresponding to the directions 

 of the secondary spectra. 



XXVII. On the Measuremerit of very large and very small 

 Alternating Currents. By Albrrt Campbell, B.A* 



AIR-core transformers, although quite inefficient for 

 ordinary lighting circuits, are yet much more valuable 

 for testing purposes than most people are aware of. By the 

 help of such transformers it is possible to extend almost 

 indefinitely the ranges of many ordinary measuring instru- 

 ments. If the secondary of an ironless transformer be kept 

 in open circuit the secondary volts are accurately proportional 

 to the primary P.D. if the frequency is constant, and hence 

 by using an electrostatic voltmeter on the secondary we can 

 transform either up or down, and thus measure voltages above 

 or below the range of the electrostatic instrument. Of course 

 the arrangement would have to be calibrated ; this might 

 sometimes be done by taking a reading for which both the 

 primary and secondary voltages lay within the range of the 

 voltmeter used. 



The above way of using an air-core transformer was sug- 

 gested to me some time ago by Mr. Hugh Erat Harrison, of 

 Faraday House. I hear since from Mr. Mather that it has 

 been also used at the Central Technical College f. 



If we attempt to measure current (in the primary) by 

 observing the voltage on an open-circuit secondary, we find 

 that for a given primary current the readings depend also on 

 the frequency . It therefore occurred to me that the secondary 



* Communicated by the Physical Society : real June 12, 1896. 



t Since writing the above I have listened with interest to the paper on 

 the " True Resistance of the Electric Arc," by Messrs. Frith and Rodgers. 

 Their beautiful application of an air-core transformer to measure a small 

 alternating current superimposed on a large direct current might, I think, 

 be also employed to separate a large alternating current from a much 

 smaller direct current. 



