1 76 THE SCIENTIFIC PAPERS OF 



In using such a table, the temperature measured by the appa- 

 ratus is found indicated at the intersection of the two columns of 

 figures, expressing the volumes of gases observed in the two tubes 

 V and V T ; these figures commence only with 40, because it is 

 not considered advisable to take an observation until at least 40 

 unit volumes of gas have been developed in each tube. Care is to 

 be taken that no leakage of gas takes place under the weighted 

 cushions, which is easily observed in allowing the depressed 

 columns to stand without lowering the reservoirs when the levels 

 between gas and liquid should remain constant. Although the 

 differential voltameter here proposed for measuring electrical re- 

 sistances not exceeding the limits of metallic and earth circuits 

 does not surpass, or even equal the Wheatstone bridge arrangement 

 for accuracy, when the latter is carefully prepared, and in the hands 

 of a skilful operator, it yet possesses advantages of its own which 

 will, I trust, recommend it to the notice of electricians. One of 



then equating the values of the resistances as given by the equations of p. 148 and 

 p. 172 respectively : 



10 (a<* + 0< + 7) = *, (17 + 2)- 2 



1-9*; 

 r-'2. 



, _. 



) + 4ft*-f ~2j8 



20 



Substituting the values , ft, 7, and remembering that the formula is calculated for 

 the absolute scale of temperature, the formula for the Centigrade scale will take 

 the following form, which is that given at the foot of the table : 

 T Centigrade 



= [{877-975 x -, + 19-070544 + 82-738226}* -9'0960553j- 274 



= { (877-975 x - + 101-80877)* -9 '0960553 ) a - 274. 

 v 



By means of this formula, the temperature of the resistance coil, which gives a 

 ratio of volumes in the voltameter tubes greater than the maxinnim given in the 

 Table can be calculated, and the constants required for the calculation have been 



