180 Mr. J. T. Bottomley. Permanent Temperature [June 11>. 



Compound conductor of small resistance, and having very large sur- 

 face exposed to the air for cooling. Between op and B a current 

 galvanometer of very small resistance is introduced for measuring the 

 whole current flowing in the branch ABC, and therefore through the 



Fio. 2. 



experimental wire BC. The branch A'B'C is a single long wire 

 having many times the resistance of the branch ABC, and many 

 times the resistance of the battery used, and thus only a small portion 

 of the current passes through it a quantity, in fact, unable to heat 

 this branch to any appreciable extent. The testing galvanometer is 

 introduced between B and B', the contact at B' being shifted about 

 till a " balance " is obtained ; and from the position of B' on the 

 scale SB the ratio can be calculated between A'B' and B'C, when the 

 eqnipotential point has thus been found. 



In experimenting, the wire BC is soldered between stout copper 

 slips for the purpose, shown in the diagram at its extremities, and it 

 resistance is taken with a very small current passing through it ; th* 

 resistance R being provided for the purpose of varying the current. 

 The current is then increased by diminishing the resistance B. When 

 this is done, the wire BC becomes heated, and its resistance increases ; 

 but the conductor AB and the branch A'B'C remain unaltered, 

 though the ratio between the parts of the latter corresponding to 

 the " balance " is changed. 



Let c be the resistance of the conductor AB ; let r t and r 2 be the 



