Intelligence and Miscellaneous Articles. 315 



circuits, in the other a length of the wire of a rheostat, of which 

 the resistance was equivalent when the needle was at zero. The 

 resistances B, B', .... of different wires submitted to experiment 

 were proportional to the lengths 1,1',.... of rheostat-wire which 

 had served to measure them ; and in order to express them as 

 functions of a given unit, it was sufficient if the ratio of the rheo- 

 stat-wire itself to this unit had been determined once for all. The 

 rheostat consisted simply of two identical, very regular platinum 

 wires, stretched parallel on a horizontal rule of two metres length. 

 These wires traversed a cork cup containing mercury, carried by a 

 cursor movable along the rule. The current, arriving through 

 the first wire, traversed the mercury and issued through the second 

 wire. On the rule was a scale of millimetres ; and shifting the 

 cursor n divisions increased or diminished the length of the circuit 

 the value of 2n. I shall not dwell upon the details, which per-r 

 mitted great precision to be attained, nor on the verifications which 

 I made of the method and of the apparatus. 



The wire under examination was soldered at each end to a copper 

 rod, then wrapped round a cylinder of pipe-clay, and, finally, 

 heated in a narrow and deep muffle which occupied the axis of a 

 large wrought-iron jar. This was placed in a gas-stove with two 

 concentric envelopes ; by introducing a suitable volatilizable sub- 

 stance, and heating to ebullition, the whole apparatus, and conse- 

 quently the wire, was brought to a fixed and known temperature. 



By determining thus the resistances of one and the same metal 

 at various known temperatures, a number of points are obtained, 

 from which the curve of the resistances can be constructed and its 

 elements calculated. 



The following are the temperatures which served for my deter- 

 minations : — 



o o 



Ebullition of water . 100 Ebullition of sulphur . 440 

 „ mercury 360 „ cadmium 860 



I made, besides, a great number of measurements below 360°, 

 the apparatus being filled with mercury and heated by a regular 

 current of gas ; the temperature was indicated by thermometers 

 placed in the muffle at different depths. 



The results obtained by the preceding method were controlled 

 and confirmed by determinations with Wheatstone's bridge, with 

 the aid of a set of resistances similar to those employed in telegraphy. 



The results are summed up in the Tables which follow. 



In the first the conductivities at zero are expressed as functions 

 of the two units which are now-a-days usually employed : — the 

 theoretic absolute unit, or dim, proposed by the British Associa- 

 tion ; and the mercury unit, adopted by M. Werner Siemens. The 

 third column gives the ratios of the conductivities to that of silver, 

 in order that the results may be compared with the well-known co- 

 efficients of MM. Becquerel, Lenz, Matthiessen, &c. 



The second Table gives the formula} of the increment of the re- 

 sistance with the temperature. This takes place regularly up to 



