Lessons in elementary physics / by Balfour Stewart.

Balfour Stewart

Date:: 1895

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

455/502 page 435

$creased ten times, the resistance is also unavoidably increased in the same proportion, and hence both numerator and de- nominator of the fraction representing intensity are multiplied by the same number. If therefore the external resistance be very small, we do not gain much by increasing the number of cells. But suppose that while the essential resistance, or b, is equal to lo, the external resistance is equal to too, then we shall have, for one cell I to and for lo cells, ^ _ io B _ E 200 20 If therefore the external resistance be great compared to the internal or essential resistance, a considerable increase in the intensity of the current is obtained by increasing the number of cells. Thus, in producing the electric light by the method in which a carbon filament is heated to incandescence by means of Grove’s cells, it is necessary that a large number should be employed if the carbon have a high resistance. Example.—A lamp has a resistance when hot of 22’5 units, and requires two units of current to raise the filament to white heat. Calculate the number of cells which should be employed, assuming each cell to have an E.M.F. of two units and an internal resistance of 'i unit. .4)iswer.—We have c= « B + R where c = 2, E = 2, B = 'i, and R = 22-5, and n is the number of cells :—hence 2 — ^ ” •I « + 22'5 and It = 25 Again, the thermo-electric current is one in which the external resistance is generally much greater than the internal or essential resistance. For in this case, the whole arrangement being metallic'without any interpo.sed fluids, the essential re- sistance is extremely small'; but in order to make use of the current, it is generally necessary to have a coil of wire con- i' E 2$

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