THE ELECTRIC RESISTANCE OF SUBSTANCES

228*. Substances according to the passage of electricity can be classified into 3:

1. The metals and their alloys

This includes some sulphurets, and other compounds containing metals, to which we must add carbon in the form of gas-coke, and selenium in the crystalline form.

In all these substances conduction takes place without any decomposition, or alteration of the chemical nature of the substance, either in its interior or where the current enters and leaves the body. In all of them the resistance increases as the temperature rises.

2. Electrolytes

The current is associated with a decomposition of the substance into 2 components which appear at the electrodes.

As a rule, a substance is an electrolyte only when in the liquid form, though certain colloid substances, such as glass at 100°C, which are apparently solid, are electrolytes. It would appear from the experiments of Sir B. C. Brodie that certain gases are capable of electrolysis by a powerful electromotive force.

In all substances which conduct by electrolysis the resistance diminishes as the temperature rises.

The third class consists of substances the resistance of which is so great that it is only by the most refined methods that the passage of electricity through them can be detected. These are called Dielectrics. To this class belong a considerable number of solid bodies, many of which are electrolytes when melted, some liquids, such as turpentine, naphtha, melted paraffin, &c., and all gases and vapours. Carbon in the form of diamond, and selenium in the amorphous form, belong to this class.

The resistance of this class of bodies is enormous compared with that of the metals. It diminishes as the temperature rises. It is difficult, on account of the great resistance of these substances, to determine whether the feeble current which we can force through them is or is not associated with electrolysis.

RESISTANCE OF METALS

On the Electric Resistance of Metals

229*.] There is no part of electrical research in which more numerous or more accurate experiments have been made than in the determination of the resistance of metals. It is of the utmost importance in the electric telegraph that the metal of which the wires are made should have the smallest attain- able resistance. Measurements of resistance must therefore be made before selecting the materials. When any fault occurs in the line, its position is at once ascertained by measurements of resistance, and these measurements, in which so many persons are now employed, require the use of resistance coils, made of metal the electrical properties of which have been carefully tested. The electrical properties of metals and their alloys have been studied with great care by MM. Matthiessen, Vogt, and Hockin, and by MM. Siemens, who have done so much to introduce exact electrical measurements into prac- tical work.

It appears from the researches of Dr. Matthiessen, that the effect of tem- perature on the resistance is nearly the same for a considerable number of the pure metals, the resistance at 100°C being to that at 0°C in the ratio of 1.414 to 1, or of 1 to .707. For pure iron the ratio is 1.645, and for pure thallium 1.458.

The resistance of metals has been observed by Dr. C. W. Siemens∗ through a much wider range of temperature, extending from the freezing point to 350°C, and in certain cases to 1000°C. He finds that the resistance increases as the temperature rises, but that the rate of increase diminishes as the tem- perature rises. The formula, which he finds to agree very closely both with the resistances observed at low temperatures by Dr. Matthiessen and with his own observations through a range of 1000°C, is

r = αT 2 + βT + γ, where T is the absolute temperature reckoned from −273°C, and α, β, γ are ∗ Proc. R. S., April 27, 1871.RESISTANCE OF METALS. 224 constants. Thus, for 1 Platinumr = 0.039369T 2 + 0.00216407T − 0.2413, Copperr = 0.026577T 2 + 0.0031443T − 0.22751, Ironr = 0.072545T 2 + 0.0038133T − 1.23971. 1 1 From data of this kind the temperature of a furnace may be determined by means of an observation of the resistance of a platinum wire placed in the furnace.

Dr. Matthiessen found that when two metals are combined to form an alloy, the resistance of the alloy is in most cases greater than that calculated from the resistance of the component metals and their proportions. In the case of alloys of gold and silver, the resistance of the alloy is greater than that of either pure gold or pure silver, and, within certain limiting proportions of the constituents, it varies very little with a slight alteration of the proportions. For this reason Dr. Matthiessen recommended an alloy of two parts by weight of gold and one of silver as a material for reproducing the unit of resistance. The effect of change of temperature on electric resistance is generally less in alloys than in pure metals.

Hence ordinary resistance coils are made of German silver, on account of its great resistance, and its small variation with temperature. An alloy of silver and platinum is also used for standard coils. 230*.] In the following table R is the resistance in Ohms of a column one metre long and one gramme weight at 0°C, and r is the resistance in centime- tres per second of a cube of one centimetre, according to the experiments of Matthiessen∗ . ∗

Phil. Mag., May, 1865.225 RESISTANCE OF ELECTROLYTES. Specific gravity Silver Copper Gold Lead Mercury Gold 2, Silver 1 Selenium at 100°C 10·50 8·95 19·27 11·391 13·595 15·218 hard drawn hard drawn hard drawn pressed liquid hard or annealed Crystalline form RrPercentage increment of resistance for 1°C at 20°C. 0·1689 0·1469 0·4150 2·257 13·071 1·6681609 1642 2154 19847 96146 10988 6 × 10130·377 0·388 0·365 0·387 0·072 0·065 1·00

It appears from the researches of Matthiessen and Hockin that the resis- tance of a uniform column of mercury of one metre in length, and weighing one gramme at 0°C, is 13.071 Ohms, whence it follows that if the specific gravity of mercury is 13·595, the resistance of a column of one metre in length and one square millimetre in section is 0.96146 Ohms.