_=358. Internal Resistance and Current Strength.=_ It is evident that the internal resistance of a cell varies with the position and size of the plates. We shall now study the effects of these changes upon the strength of the current. [Illustration: Fig. 108.] =EXPERIMENT 142. Having a cell with LARGE PLATES, to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.= _Apparatus._ Galvanoscope, G V; materials for simple cell (Exp. 110); connecting wires. Arrange as in figure 108, omitting the wooden cross-piece. =359. Directions.= (A) Connect the wires with the 5-turn coil of G V, which has but little resistance. Have the tumbler nearly full of dilute acid to get the effect of large plates; that is, the current has a large liquid conductor to pass through in the cell, and the _internal_ resistance will be small. G V should be properly placed N and S. (B) Place the copper and zinc plates as far apart as possible in the acid, and press them against the bottom of the tumbler. Note the reading of G V. It is not necessary to take readings with reversed current. (C) Still pressing them against the bottom of the glass, to keep the same amount of surface under acid, slowly bring them near each other and watch the needle. (D) Hold the plates about an inch apart, and against the bottom, and note the reading of G V. Slowly raise the plates, keeping them the same distance apart until they are out of the acid. Watch the action of the needle. Make a note of your readings in degrees and write your conclusions. Does a change in internal resistance affect the strength of the current? =EXPERIMENT 143. Having a cell with SMALL PLATES to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.= _Apparatus._ Same as in Exp. 142, the acid, however, being but 1 in. deep in the tumbler; that is, we have the effect of a cell with small plates, each being about 1 in. by 1/2 in. =360. Directions.= (A) Repeat (B) and (C) of Exp. 142, recording the reading of G V in each case. (B) Compare the results with those of Exp. 142, remembering that the _internal_ resistance is larger than before. Is the current as strong with small plates as with large plates when the external resistance is small? When the external resistance is small (the 5-turn coil, for example), should the cell have a high or low internal resistance to produce the greatest effect upon the needle? [Illustration: Fig. 109.] =EXPERIMENT 144. To find whether the changes in current strength, due to changes in internal resistance, are as great when the external resistance is large, as they are when the external resistance is small.=