_Apparatus._ Same as for Exp. 142, 143, also the rheostat containing the two meters of G-s wire (Exp. 121). =361. Directions.= (A) Arrange as in Fig. 109, the external resistance being 2 meters of No. 30 G-s wire in series with G V. The 2-F C in the Fig. is replaced, however, by the simple cell as in Exp. 143. (B) Find the effect upon the strength of the current of moving the plates about when but 1 in. of acid is in the tumbler. (C) Nearly fill the tumbler with acid and repeat (B), taking readings with plates near each other and as far apart as possible. Lift them nearly out of the acid and take the reading. (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, 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 1/4 ampere, as R + r = 4. The change in C, then, is the difference between 1/3 and 1/4; 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. Increasing r from 2 to 3, as before, etc., we find that C = 1 divided by 203 = .00492 ampere. The above shows clearly (A) that the value of C is changed considerably by changes in r when R is _small_, and (B) that changes in r produce very slight changes in C when R is _large_. Review your results of Exps. 142-144. (See Elementary Electrical Examples.) _=363. Arrangement of Cells and Current Strength.=_ We have seen that internal resistance affects current strength. In joining cells, then, attention must be given to the internal resistance as well as to the E. M. F. of the combination.