CHAPTER XVII. ELECTROMOTIVE FORCE 117 Electromotive Force.--Unit of E. M. F.; The Volt.--Exp. 116, To see whether the E. M. F. of a cell depends upon the materials used in its construction.--Discussion.--Electromotive Series.--Exp. 117, To see whether the E. M. F. of a cell depends upon its size.--Discussion. CHAPTER XVIII. ELECTRICAL RESISTANCE 120 Resistance.--Exp. 118, To study the general effect of "resistance" upon a current.--External Resistance; Internal Resistance; Unit of Resistance; The Ohm.--Resistance Coils; Resistance Boxes.--Simple Resistance Coil.--Exp. 119, To test the power of various substances to conduct galvanic electricity.--Conductors and Nonconductors.--Exp. 120, To find the effect of sulphuric acid upon the conductivity of water.--Internal Resistance.--Exp. 121, To find what effect the length of a wire has upon its electrical resistance.--Discussion.--Exp. 122, To find what effect the size (area of cross-section) of a wire has upon its electrical resistance.--Discussion.--Exp. 123, To compare the resistance of a divided circuit with the resistance of one of its branches. Discussion.--Exp. 124, To study the effect of decreasing the resistance in one branch of a divided circuit.--Current in Divided Circuits. CHAPTER XIX. MEASUREMENT OF RESISTANCE 130 Exp. 125, To study the construction and use of a simple Wheatstone's Bridge.--The Simple Bridge.--Equipotential Points.--Example.--Exp. 126, To measure the resistance of a wire by means of Wheatstone's Bridge; the "bridge method."--Allowances for Connections.--Exps. 127-137, To measure the resistances of various wires, coils, etc., by the "bridge method."--Table.--Exp. 138, To study the effect of heat upon the resistance of metals.--Effect of Heat upon Resistance.--Exp. 139, To measure the resistance of a wire by the "method of substitution."--Simple Rheostat.--Exp. 140, To measure the E. M. F. of a cell by comparison with the two-fluid cell.--Exp. 141, To measure the internal resistance of a cell by the "method of opposition." CHAPTER XX. CURRENT STRENGTH 142