Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    453/502 page 433
    of its substance and its temperature. The following table, due to Matthiessen and von Bose, gives the conductivity of various metals compared with silver. T.\ble No. 44.—Electric Conductivities. at 0° C. at 100* C Name of Metal. (Silver at o° C. (Silver at o® C. = 100) = 100) Silver (hard-drawn) . . .... 100*00 . . • 71‘56 Copper (hard-drawn) . . ■ 99'95 ■ • • 7°'27 Gold (hard-drawn) . . . .... 77-96 . . ■ 55'9° Zinc .... 29*02 . . . 20-67 Cadmium . . 23-72 . . • 1677 Tin .... 12-36 . . . 8-67 Lead .... 8-32 . . 5-86 Arsenic .... 4-76 ■ 3‘33 .'Antimony . . . . 4-62 . . ■ 3'26 Bismuth . . . . 1-245 . 0-878 It has been remarked by Principal Forbes that metals follow one another in the same order, whether as conductors of heat or of electricity, and this is borne out by comparing the above table with that of Art. 219. Tait has furthermore shown that if two specimens of the same metal vary in their electrical conductivity, they vary in the same manner as regards their thermal conductivity. Finally, it would appear that both the electric and the thermal conductivity of metals are diminished by increasing their temperature, in such a manner that the conductivity varies inversely as the absolute temperature (Art. 247). 2nd. Its resistance varies inversely as its cross section ; that is to say, a wire with a large cross section offers much less resistance to the passage of a current than one with a small section. 3rd. Its resistance is proportional to its length ; that is to say, if a current has to pass through two miles of wire it will be twice as much resisted as if it had to pass through one mile. Expressing these laws in a formula, we have for a wire where r is the resistance, p the specific resistance, l. the length, and A the .area of cross section. E I-
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