Meters and other Electrical Instruments. 3 



producing eddy-currents. The thermopile consistt-d of ten 

 pairs of thin iron and nickel wires each 7 millim. long. 

 These metals were chosen as their thermo-electric lines are 

 far apart, and almost parallel to one another. 



Some years ago the writer showed (Proc. Roy. Soc. Edin. 

 July 1887) that a thermopile used thus could give a fairly 

 accurate measurement of the current through the resistance- 

 wire, the ultimate deflexion being proportional to the square 

 of this current. Hence, for a given frequency, the mean 

 square of the P.D. at the terminals of the search-coil was 

 proportional to the deflexion. Each time it was used the 

 combination was calibrated in the following manner : — A 

 measured current of 1 ampere from the alternating supply 

 circuit used was passed through a non-inductive resistance of 

 0*2 ohm, and the resulting P.D. of 0*2 volt was applied to 

 the ends of the resistance R. From the observed deflexion 

 of the galvanometer the mean square of the volts per division 

 was found*. In all cases when the search-coil was in circuit 

 the frequency n was observed, being measured by a frequency- 

 teller. If the resistance of the search-coil be negligible, and 

 if the flux follows the sine law, we have 



r 2 =:10- 8 x27r^]Sr 2 B^, 

 where 



v 2 = voltage shown by galvanometer, 



n = frequency, 



N 2 = number of turns in search- coil, 



s 2 = area of search-coil, 

 and 



B = Vuieaii square B. 



The other method, which was by means, of a telephone, 

 was a rougher way, and will be described below. 



In Table I. are given some of the results obtained in the 

 case of the simpler instruments, the third column giving the 

 resistance of the instrument, the fourth column its full load, 

 and the fifth the mean value of B at that load. As far as 

 possible the positions from which the mean value of B were 

 obtained were chosen so as to give an idea of the working 

 flux-density, and except where otherwise stated the values 

 given refer to full load. 



* The thermopile method of measuring' small voltages is now in lise 

 in the German Reichsanstalt. 



B2 



