374 



TENTH ANNUAL REPORT OF 



metals have entirely changed their respective positions in the order o 

 conductive capacity : lead has become the worst conducting metal ; 

 platinum has gone above iron ; brass conducts better than tin, which, 

 at 0°, is above it. At 200° the series is relatively the same as at 

 100°, though here copper and gold have become nearly equal ; so that 

 gold, at a yet higher temperature, must be a better conductor than, 

 copper, 



In reference to the method by which Lenz arrived at the above re- 

 sults, we have a few remarks to make. The current which he used 

 was magneto-electrical, in the closing circuit of which a multiplier was 

 inserted alternately with and without the wire to be determined. 

 This wire was coiled spirally, yet so that the single coils did not touch, 

 and it was plunged in an oil bath, kept at a constant temperature by a 

 spirit-lamp. The conductive capacity of the wire was now determined 

 for a series (mostly 10 to 15) ot different temperatures of the oil bath, 

 and then by means* of the different relative values of the conductive 

 capacity g and the temperature t, the probable values of the constant 

 factors of the equation, 



g :=z a -\- ht -\- ci~, 



were found. In this manner the following equations for computing 

 the conductive capacity of different metals were obtained : 



For Silver g = 136.25 — 0.4984 t + 0.000804 t^ 



Copper g =z 100.00 — 0.3137 t + 0.000437 t^ 



79.79 — 0.1703 t -f 0.000244 t^ 

 30.84 — 0.1277 t -f 0.000273 ^^ 

 29.33 — 0.0517 t + 0.000061 t~ 

 17.74 — 0.0837 t + 0.000150 t^ 

 14.62 — 0.0608 t + 0.000107 t^ 

 14.16 — 0.0389 t + 0.000066 t^ 



Gold gz=z 



Tin g = 



Brass g z= 



Iron g =. 



Lead g = 



Platinum g =. 



These formulas, by which the above table was computed, accord very 

 well with the observations. 



E. Becquerel has also investigated the relation of the conductive 

 capacity of metals to temperature. 



The method by which Becquerel maintained his wires at a high tem- 

 perature is as follows : The metallic wire to be used in 

 the experiments is wound on a glass tube C D, Fig. 21, 

 one centimetre in diameter and five or six centimetres 

 in length, so that the single coils do not touch each other. 

 If the wire should be more than one layer, it must be 

 covered with silk, and then the second layer of coils 

 wound on the tube. To prevent the coils from unroll- 

 ing, they are fastened with silk. Both ends of the wire 

 are now fastened to the lower ends of the thick copper 

 rods a h, whose resistance may be disregarded. One of 

 tiie rods, namely, «, is fastened to the upper end of the 

 glass tube C D ; the other, h, passes down into the tube. 

 The coil, with its wrappings, is now placed in a test 

 tube filled with oil. The two rods a and h pass through 

 two small openings made in the cork A A', which holds 

 C D in the middle of the oil. A thermometer with a 

 long bulb serves for taking the temperature of the oil. 



