-rheostat 



6.0 



a 



11.7 



r' = 



5.7 



T =: 



20.8 



9 = 



6.4 



R + p = 



18.4 



P = 



6.0 



. R = 



12.4 



R = 



12.7 



II. ELECTRICAL RHEOMETRY. 37 



Observation. — In practice, it is not necessary to keep any note of the primitive 

 deviation of the galvanometer in order to determine the resistance of the circuit, 

 that deviation being only useful to ascertain the constancy of the current, for 

 which purpose any deviation observed in similar circumstances may be employed. 



We shall subjoin an example to illustrate the method of conducting the 

 experiment. 



Example. 



TURNS. 



Divided current — galvanometer 76° — rheostat 

 Current not divided " do. 



Turns added .... 

 Resistance of the dividing coil 

 Do. of the galvanometer 



Total resistance .... 

 Subtracting p . . . , 



Constant resistance 

 Another experiment afforded 



The small difference, 0.3 turns, might easily be produced by a small error in the 

 values of r or g, multiplied by the great resistance used the second time, which 

 was = 98 turns. This also proves that it is not indifferent to introduce long 

 resistances or short ones, but, for greater accuracy in practice, it is necessary to 

 avoid all unnecessary lengths of wire. 



As a source of error may arise from the multij)lication of the slight inaccuracy 

 in the values of r and g, so we have taken every care to eliminate these errors by 

 the following means : A deviating wire whose resistance was equal to that of the 

 galvanometer was used. To obtain this with greater accuracy, we adopted in the 

 galvanometer a wire, exactly of the same length and diameter as that which was 

 coiled on the cylinder r, Fig. II. By so doing, a part of the calculations is spared, 

 and errors of measure are avoided, since formula (1) becomes R + p ^ r^ + g, and 

 so the number of turns r^ expresses the total resistance of the circuit, minus that 

 of the galvanometer. 



But when it is not convenient to change the wire of the galvanometer, and there 

 is no other galvanometer which can be used for determinin"; the resistance of the 



Now our plan of experiments requires that, after suppressing the division of the current, we should add 

 Ss many turns of the rheostat as may be necessary to reduce the intensity i^ to F^: let therefore r' be the 

 turns wanted ; from the formula (»«) we shall have 



F = ^ 



' Ji' + 'J + r' 



From the equality of the 2d members of these two last equations we have 



i?' = i? + P = !Zl 

 'J 

 to which, adding the resistance of the galvanometer t/, we shall have the constant resistance of the circuit 

 expressed, as in the text. 

 10 



