142 



Mr. Julius Frith on 



A method devised by Mr. Lees was next tried, in which 

 the primary of a transformer was put in series with the arc 

 lamp, dynamo, and galvanometer. On a current being made 

 or broken in the secondary of the transformer, a kick should 

 be observed on the galvanometer which is only dependent 

 on the resistance in circuit and not on any back E.M.F. 

 should such exist in the arc. It was found, however, that 

 owing to the continual minute variations in the current 

 through the arc, this kick, though observed, was rendered 

 unavailable for purposes of measurement. 



Then methods were tried involving the measurement of 

 resistance between two points in the arc circuit which could 

 be kept at the same potential. 



Fig. 3 shows the simplest arrangement for this 

 purpose, and out of it the final 

 arrangement was evolved. It is 

 analogous to Von Lang's method. 

 D / and D„ are two shunt wound 

 dynamos in series with a resist- 

 ance R and an arc lamp. By 

 an adjustment of R the points 

 A and B can be brought to the 

 same potential, as shown by the 

 high resistance galvanometer G. 

 When this is the case the 

 resistance between them is 

 measured by the Wheatstone's 

 bridge W. 



This gives the resistance of R 

 and one dynamo in parallel with 

 the arc and the other dynamo, from which the resistance of 

 the arc can be obtained. 



First it was found that a continuous current could not 

 be used for the testing circuit, for any slight deviation from 

 equality of potential at A and B caused a current to flow 



Fig. 3. 



