(D) Still increase the external resistance of the circuit by adding coils of wire or the meter of No. 28 G-s wire and repeat. Is the strength of the current greatly affected by slight changes in the internal resistance when the external resistance is large?
362. Discussion. We shall study, by means of figures, how changes in internal resistance affect the strength of the current.
Let R stand for the total external resistance of a circuit,[147] and r for the total internal resistance of the cell or cells; ohm's law, then, will be expressed by
C = E / (R + r)
EXAMPLE. Let us take a circuit (A) when the external resistance, R, is small, and (B) when R is large compared with r, E being taken as 1 volt in both cases.
(A) Let R = 1, and r = 2; substituting these values in the formula above, we have:
C = 1 / (1 + 2) = 1 / 3 = .33+ ampere.
Now let the internal resistance, r, be slightly increased from 2 to 3 ohms; the value of C then becomes ¼ ampere, as R + r = 4. The change in C, then, is the difference between ⅓ and ¼; and this expressed in decimals becomes .33 - .25 = .08 ampere.
(B) Let R = 200 ohms, and r = 2 ohms as in (A). Substituting these values we have,
C = 1 / (200 + 2) = 1 / 202 = .00495 ampere.