a Conductor the Measure of the Current passing. 355 



All these experiments were directed to the establishment of 

 a relation between the resistance of the conductor and the 

 direct current passing. The connexions are shown in fig. 2, 

 where R is the conductor under test, shown, e. c/., as an 

 incandescent lamp, S is a standard resistance of known value, 

 B is a regulating resistance, E a storage-cell, and C a standard 

 cell. The potential-differences of R, S, and the E.M.F. of C 

 were compared on the potentiometer P for a series of different 

 currents obtained by regulating B; and from the three readings 

 the current through and the resistance of R are calculated. 

 In each case there are four leads to the resistance, two carry- 

 ing the current, and two which carry no current connected 

 to the potential points, and the resistance measured is in every 



■Fig. 2. 



O 



o 



R 



Y 



qHo__B__o}-J 



Co 



i-h 



"DO O O — ^^ O O 



m o © o 



POTENTIOMETER 



case that between the potential points. The potentiometer 

 lends itself admirably to experiments of this class, since all 

 the data are obtained from adjustments on one calibrated 

 wire, while the whole of the conductor whose resistance is 

 measured may be inside the exhausted vessel, so that the 

 leading-in wires are not involved. With a bridge the current 

 Could not be measured with the resistance, while the resistance 

 of leadino--m wires would have to be taken into account. 

 I 6. The resistance of a 50 volt, 8 c.p. Stearn lamp was 

 measured with a small testing-current immersed in a paraffln- 

 bath warmed up to 56° C. to obtain an idea of the change of 

 resistance with temperature. The mean change over a range 

 of 40° C. was — *0224 per cent, per degree. In the resistance 

 was included in this case that of the leading-in wires ; and 



