=346. Directions.= (A) Be sure that 2-F C gives a constant current. Take the reading of G V without the rheostat in the circuit; that is, with wires, 2 and 1, joined directly. The deflection should be 50 or 60 degrees at least, and be constant. (B) Attach a small piece of copper to the end of 1, and firmly rub it along upon the G-s wire, thus introducing resistance into the circuit, until the deflection is, say, 60° (50 or 55 degrees will do). Note the length of G-s wire used and call it (B). (C) Gradually add more resistance by moving the end of 1 along until the deflection is 50°, 10 degrees less than before. (If the original was 50° make the new 40°). Call the number of cm. of wire used (C). (D) Replace 2-F C with the dry cell D C. Add resistance, as before, until G V indicates a deflection of 60°, being careful not to keep the circuit closed long enough to partially polarize D C. Make 2 or 3 trials, allowing D C to rest a few minutes between each. Call the number of cm. of G-s wire used (D). (E) Again add more resistance, as in (C), until the deflection is reduced to 50°. Call the length used (E). =347. Calculation.= It is known that resistances that are able to reduce the strength of the currents equally are proportional to the electromotive forces; that is, the electromotive forces of the two cells are to each other as the two resistances necessary to produce equal changes in the deflections, which, of course, indicate equal changes in the strength of the currents. Since the resistances used in the two cases are directly proportional to the lengths used, we have: Length (C-B): Length (E-D):: E. M. F. of 2-F C: E. M. F. of D C. Substitute the values found and find the E. M. F. of D C. =EXPERIMENT 141. To measure the internal resistance of a cell by the "method of opposition."= _Apparatus._ All the apparatus of Exp. 126. Two simple cells (§ 275), the plates of which should be of the same size, the same distance apart, and immersed in acid to the same extent in both. The acid in both should be of the same strength. =348. Directions.= (A) Connect the two cells in opposition, so that no current will be generated by them, and so that the two can be treated as a dead resistance. Do this by joining the two zinc plates by a wire with connectors, and use wires to connect the copper plates to the bridge like any other unknown resistance. (B) Measure the resistance of the two by the regular bridge method, allowing for wires used for connections. One-half of the resistance found will give the internal resistance of one cell. (See Note.)