For pure metals the resistance increases considerably with increase of temperature. With certain alloys the change is so slight as to be negligible. In some alloys, and in carbon and insulating materials, the resistance falls with increase of temperature.
Measurement of Resistance.—Low resistances can most conveniently be measured by a fall of potential method, based on the relationship R = V/I. The current may be read by an ammeter, and the potential difference by a low-reading voltmeter (see Electrical Measuring Instruments). Where greater accuracy is required, a constant current is sent through the resistance to be measured, and also through a known standard resistance of about the same value. A sensitive galvanometer (see Galvanometer) is used to compare the P.D. across the unknown resistance with that across the standard. Since the current is the same through both, the resistances will be proportional to the galvanometer deflections, and from the known value of the standard resistance the value of the unknown resistance can be calculated. Resistances of moderate value are best measured by a Wheatstone Bridge, or one of its modifications (see Wheatstone Bridge).
A substitution method is more suitable for high resistances. A galvanometer is connected in series with a standard high resistance and a steady source of E.M.F. The deflection is noted. The unknown resistance is now substituted for the standard, and the new deflection noted. Provided the resistance of the galvanometer and other parts of the circuit is negligible in comparison with the resistance to be measured, the resistances are inversely as the deflections. The unknown resistance is, therefore, equal to the ratio of the first to the second deflection multiplied by the value of the standard resistance. For insulation tests on installations, direct-reading instruments are frequently used (see Ohmmeter).
Effects of an Electric Current.—When a current flows in a conductor, the temperature of the conductor is raised. This is due to the power dissipated on account of the resistance of the conductor. The power dissipated is equal to I2R watts, and by giving suitable values to I and R any required amount of heat per second can be obtained. This heating effect of the current is made use of in electric lighting, electric heating and cooking, in electric furnaces, and in certain electro-medical appliances.
If a magnetic needle is brought near a conductor carrying a current, it will be found to be deflected. This is due to the magnetic field, which is always associated with an electric current. This electro-magnetic effect is of the utmost practical importance (see Electro-magnetism; Generator; Electric Motors).
When a current is passed through a conducting liquid, such as a solution of a metallic salt or a salt in a fused state, chemical action takes place. The behaviour of such a conductor is entirely different from that of a metallic conductor, since a current can flow in it only if chemical dissociation takes place (see Electrolysis).
Practical use of electrolysis is made in electroplating, the production of electrotype blocks for printing, the refining of copper, and the production of metallic sodium and potassium. Electrolysis is also used as a means of storing electrical energy in a chemical form (see Secondary Cell).
An electric current may be constant in direction (direct current), or may alternate in direction with a certain frequency (alternating current). Alternating currents have advantages for the transmission of large amounts of power over considerable distances (see Electric Power Transmission and Distribution), and may be used for electric lighting and the operation of electro-dynamic machines and apparatus (see Electric Motors).
Bibliography: B. Kolbe, Introduction to Electricity; S. P. Thompson, Elementary Lessons in Electricity; Starling, Electricity and Magnetism; Poynting and Thomson, Electricity; W. E. Ayrton, Practical Electricity; C. R. Gibson, Electricity of To-day; Clerk-Maxwell, Electricity and Magnetism; E. E. Brooks and A. W. Poyser, Magnetism and Electricity.