290 Messrs. J. H. Gray and J. B. Henderson. 



and Synthetical Attempts to ascertain the canse of the Differences of 

 Electrical Conductivity discovered in Wires of nearly Pure Copper," 

 and " On the Electric Conductivity of Commercial Copper of various 

 kinds," printed in his collected papers, the " weight specific resist- 

 ance " is always used. Several of the most eminent authorities have 

 from time to time signified their disapproval of the volume unit, but 

 it still continues to be most generally used. Why this is so does not 

 seem clear, for surely a much more definite idea of a metal is 

 obtained when its density is taken into account as in the weight 

 unit ; and if, for certain purposes, the volume unit may be con- 

 venient, this can easily be found from a knowledge of the density. 

 It has been conclusively established, first by Lord Kelvin, and after- 

 wards by several investigators, that the volume specific resistance 

 always increases with the decrease of density, and therefore the 

 weight resistance, since it includes the density, will not change so 

 much. The weight unit is not, however, quite constant, as the 

 results of this investigation show, but the changes, at least for 

 copper, iron, and steel, are very small. As a matter of fact, for 

 these metals the volume specific resistance does not change very 

 much either, as the density is practically constant for any mechanical 

 treatment. 



The method used for the test of change of specific resistance was a 

 slight modification of that known as Thomson's (Lord Kelvin) 

 Double Bridge Method (" New Electrodynamic Balance for Resist- 

 ance of Short Bars or Wires," ' Phil. Mag.,' 4th Series, vol. 24, 1862, 

 p. 149). Diagram 2 is a sketch of the arrangements. A length of 

 wire, PCDB, of about 10 metres, was fixed about its middle point 

 round two strong bolts, C and D, which were fixed firmly near the 

 ceiling in a pillar of the physical laboratory. At the points P, Q, R, 

 the ends of 150 ohms resistances, r 1} r 2 , r 3 , were neatly soldered, the 

 other ends of these resistances from Q and B being brought to one 

 terminal, H, of a Thomson's mirror galvanometer G, of resistance 

 5380 ohms, the end of the resistance from P being connected to the 

 other terminal K of the galvanometer. From K another 150 ohms 

 resistance, r 4 , was carried to a sliding contact, B. The four resist- 

 ances of 150 ohms each were inserted so that practically all the 

 current from the battery L would flow in the circuit PCDB, and 

 thus any movement of B will not sensibly disturb the distribution in 

 this circuit. As will be at once seen, if the resistance of the wire 

 PQ is equal to that of BR, there will be no deflection in the galvano- 

 meter. The wire PQ was left unaltered, and served as a standard 

 of comparison for BR, which was subjected to successive stretchings 

 by means of weights, W. Half-millimetre scales, S, S, fixed imme- 

 diately behind the points R and B, enabled the readings of length, 

 BR, to be accurately taken. 



