Strength of Current.--Unit of Current Strength; The Ampere.--Measurement of Current Strength.--The Tangent Galvanometer.--The Ammeter.--The Voltameter.--Unit of Quantity; The Coulomb.--Electrical Horse-power; The Watt.--Ohm's Law.--Internal Resistance and Current Strength.--Exp. 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.--Exp. 143, Same as Exp. 142, but with small plates.--Exp. 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.--Discussion, with examples.--Arrangement of Cells and Current Strength.--Cells in Series.--Cells Abreast.--Exp. 145, To find the best way to join two similar cells when the external resistance is small.--Exp. 146, To find the best way to join two similar cells when the external resistance is large.--Best Arrangement of Cells. CHAPTER XXI. CHEMICAL EFFECTS OF THE ELECTRIC CURRENT 151 Chemical Action and Electricity.--Electrolysis.--Exp. 147, To study the electrolysis of water.--Composition of Water.--Electromotive Force of Polarization.--Exp. 148, To coat iron with copper.--Exp. 149, To study the electrolysis of a solution of copper sulphate.--Electroplating.--Exp. 150, To study the chemistry of electroplating.--Discussion.--Electrotyping.--Voltameters.--Exp. 151, To study the construction and action of a simple "storage" cell.--Secondary or Storage Cells. CHAPTER XXII. ELECTROMAGNETISM 158 Electromagnetism.--Exp. 152, To study the lines of force about a straight wire carrying a current.--Ampere's Rule.--Lines of Force About Parallel Wires.--Exp. 153, To study the lines of force about a coil of wire like that upon the galvanoscope.--Exp. 154, To study the magnetic field about a small coil of wire.--Coils.--Polarity of Coils.--Exp. 155, To test the attracting and "sucking" power of a magnetized coil or helix.--Exp. 156, To find whether a piece of steel can be permanently magnetized by an electric current.--Exp. 157, To study the effect of a piece of iron placed inside of a magnetized coil of wire. CHAPTER XXIII. ELECTROMAGNETS 165