272 Mr Griffiths, The Calibration of a Bridge Wire. [Feb. 11, 



part. A slight modification of the method I have here described 

 enables a rough determination of the evenness of the wire to be 

 made before placing it in situ, and thus much trouble may be 

 saved, if the wire is found to be so unequal as to lead to its 

 rejection. 



In this case the contact makers d^ and P^ can be fixed by 

 shellac to a slip of ebonite at any convenient distance apart. They 

 are then brought in contact with the bridge wire and d^ adjusted 

 on the other circuit, as before described. If the ebonite is then 

 slipped along AB so that d^ and P^ are continually in contact 

 with the bridge wire the swings of the galvanometer will serve 

 as a sufficient indication of the equality of the different portions 

 of the wire *. 



Note, February 25th, 1895. 



Since the communication of the preceding paper to the Philo- 

 sophical Society, my attention has been directed to the fact that 

 the account of a somewhat similar mode of calibration, due to Von 

 Helmholtz, was published by Giese in Wied. Ann., Vol. xi. p. 440, 

 1880. The method there described is dependent on the fall 

 of potential down the wires of two separate circuits. When I 

 made my communication I was, of course, unaware that this 

 principle, obvious as it is, had been previously applied. The mode 

 of application is, however, entirely distinct from the method I 

 have adopted, which is free from many causes of error inseparable 

 from that described by Giese. The chief points of difference are 

 as follows. Suppose a third point of contact m between d^ and Pj , 

 and let this point m be connected with a key in such a manner 

 that the wires leading from the galvanometer and from d^ can be 

 placed first in connection with cZ, and m and secondly in connec- 

 tion with m and P^. If the deflection of the galvanometer is the 

 same in both cases, then the resistance of d^m is equal to the 

 resistance of mP^, and thus by slightly shifting m the resistance 

 d^P^ can be bisected. 



Hence it is obvious that changes in the resistance of the 

 contacts at d^m and Pj will affect the galvanometer-deflection as 

 also will changes in the resistance of the key-contacts by which 

 the connections are shifted. Also changes in temperature of the 

 galvanometer would affect the resistance of its coils, and thus 

 influence the swing. Again, as pointed out by Giese himself, any 

 falling off in the electromotive force of the cells S^ and S^ will 



* In the discussion on this communication it was pointed out by Prof. Ewing 

 that this method would be a convenient one to use in those cases where the wire 

 was to be made approximately uniform by scraping. 



