Third Class.—Dielectrics

130.] The third class of bodies has an electric resistance so much greater than that of metals, or even of electrolytes, that they are often called insu- lators of electricity. All the gases, many liquids which are not electrolytes, such as spirit of turpentine, naphtha, &c., and many solid bodies, such as gutta-percha, caoutchouc in its various forms, amber and resins, crystallized electrolytes, glass when cold, &c., are insulators.

SPECIFIC INDUCTIVE CAPACITY

They are called insulators because they do not allow a current of electricity to pass through them. They are called dielectrics because certain electrical actions can be transmitted through them. According to the theory adopted in this book, when an electromotive force acts on a dielectric it causes the electricity to be displaced within it in the direction of the electromotive force, the amount of the displacement being proportional to the electromotive force, but depending also on the nature of the dielectric, the displacement due to equal electromotive forces being greater in solid and liquid dielectrics than in air or other gases.

When the electromotive force is increasing, the increase of electric dis- placement is equivalent to an electric current in the same direction as the electromotive force. When the electromotive force is constant there is still displacement, but no current. When the electromotive force is diminishing, the diminution of the electric displacement is equivalent to a current in the opposite direction.

131.] In a dielectric, electric displacement calls into action an internal electromotive force in a direction opposite to that of the displacement, and tending to reduce the displacement to zero. The seat of this internal force is in every part of the dielectric where displacement exists. To produce electric displacement in a dielectric requires an expenditure of work measured by half the product of the electromotive force into the electric displacement. This work is stored up as energy within the dielectric, and is the source of the energy of an electrified system which renders it capable of doing mechanical work.

The amount of displacement produced by a given electromotive force is different in different dielectrics. The ratio of the displacement in any dielec- tric to the displacement in a vacuum due to the same electromotive force is called the Specific Inductive Capacity of the dielectric, or more briefly, the Dielectric Constant. This quantity is greater in dense bodies than in a so-called vacuum, and is approximately equal to the square of the index of refraction. Thus Dr. L. Boltzmann∗ finds for various substances, ∗ [Pogg. Ann. CLI. (1874), p. 482.]123 PROPERTIES OF A DIELECTRIC. Sulphur (cast) Colophonium Paraffin Ebonite (Hartgummi) √D. 1·960 1·597 1·523 1·775 D. 3·84 2·55 2·32 3·15 Index of refraction. 2·040 1·543 1·536 For a sphere cut from a crystal of sulphur Boltzmann finds D by electrical experiments for the three principal axes, and compares them with the results as calculated from the three indices of refraction. By electrical experiments By optical measurements D1 = 4·773 D1 = 4·596 D2 = 3·970 D2 = 3·886 D3 = 3·811 D3 = 3·591 {Sitzungsb. (Vienna), 9 Jan., 1873.} 132.] Schiller (Pogg. Ann. CLII. 535) ascertained the time of the electri- cal vibrations when a condenser is discharged through an electromagnet. He finds in this way the following values of the dielectric coefficients of various substances, and compares them with those found by Siemens by the method of a rapid commutator. Ebonite (Hartgummi) Pure rubber Vulcanized grey, do. Paraffin, quick cooled, clear „ slow cooled, milk white „ another specimen Straw coloured glass „ „ White mirror glass Schiller. 2·21 2·12 2·69 1·68 1·81 1·89 2·96 3·66 5·83 Siemens. 2·76 2·34 2·94 1·92 2·47 4·12 μ2 .μ. 2·251·50 2·19 2·341·48 1·53 6·34 P. Silow {Pogg. Ann. CLVI (1875), [p. 395]}∗ finds for oil of turpentine D = 2·21 √D = 1·490 μ∞ = 1·456. Faraday did not succeed in detecting any difference in the dielectric con- stants of different gases. Dr. Boltzmann† however has succeeded by a very ∗ † [See also CLVIII. (1876), pp. 306 et sqq.] [Pogg. Ann. CLI. (1875), p. 403.]124 DISRUPTIVE DISCHARGE. ingenious method in determining it for various gases at 0°C, and at one at- mosphere pressure. Air Carbonic Acid Hydrogen Carbonic Oxide Nitrous Oxide Olefiant Gas Marsh Gas D. 1·000590 1·000946 1·000264 1·000690 1·000994 1·001312 1·000944 √D. 1·000295 1·000473 1·000132 1·000345 1·000497 1·000656 1·000472 μ. 1·000294 1·000449 1·000138 1·000340 1·000503 1·000678 1·000443