The galvanometer being so adjusted that a current of definite strength through one of the coils gives a definite deflection of the needle, the amount of leakage expressed in terms of the insulation resistance of the wires is given by the formula.
For the purpose of avoiding calculation, tables are provided showing the values of the total insulation according to the formula, corresponding to various values of d.
In this way communication was established from both sides on the 16th of August, but it did not continue long, for the insulation had been ruined by Whitehouse's treatment, and after the 20th of October no signals could be got through.
Oliver Heaviside showed mathematically that uniformly-distributed inductance in a telephone line would diminish both attenuation and distortion, and that if the inductance were great enough and the insulation resistance not too high the circuit would be distortionless, while currents of all frequencies would be equally attenuated.
The last elaboration of the insulated slip water-bottle by Ekman, Nansen and Pettersson has produced an instrument of great perfection, in which the insulation is effected by layers of water between a series of concentric ebonite cylinders, all of which are closed both above and below when the apparatus encloses a sample, and each of which in turn must be warmed considerably before there is any rise of temperature in the chamber within.
OHMMETER, an electrical instrument employed for measuring insulation-resistance or other high electrical resistances.
Suppose it is desired to measure the insulation-resistance of a system of electric house wiring; the ohmmeter circuits are then joined up as shown in fig.
On setting the dynamo in operation, a current passes through the shunt coil of the ohmmeter proportional to the voltage of the dynamo, and, if there is any sensible leakage through the insulator to earth, at the same time another current passes through the series coil proportional to the conductivity of the insulation of the wiring under the electromotive force used.
The Evershed and Vignoles form of the instrument is much used in testing the insulation resistance of electric wiring in houses.
Of the ends of the insulator and the current flowing through it, that is, by its insulation resistance.
In this case one terminal of the battery is connected to the earth, and the other terminal is connected through the galvanometer with the copper wire, the insulation of which it is desired to test.
In 1140 a Benedictine monastery was founded here by Ralph Boteler of Oversley, and received the name of the Church of Our Lady of the Isle, owing to its insulation by a moat meeting the river Arrow.
The great advantage of this air insulation is that the electrostatic capacity of the wires is low (about one-third of that which would be obtained with gutta-percha insulation), which is of the utmost importance for high-speed working or for longdistance telephonic communication.
Total insulation resistance of looped lines = 2 R(D/d - 1); in which R is the total resistance of the looped wires, including the resistance of the two coils of the galvanometer, of the battery, and of the two resistance coils r and r' (inserted for the purpose of causing the leakage on the lines to have a maximum effect on the galvanometer.
The climatic conditions in the British Islands are such that it is not possible to maintain, in unfavourable weather, a higher standard than that named, which is the insulation obtained when all the insulators are in perfect condition and only the normal leakage, due to moisture, is present.
The insulation is again tested, and if no fault is discovered the served core is passed through the sheathing machine, and the iron sheath and the outer covering are laid on.
Hence the deflection of the needle is proportional to the insulation resistance, and the scale can be graduated to show directly this resistance in megohms.
When the price of aluminium is less than double the price of copper aluminium is cheaper than copper per unit of electric current conveyed; but when insulation is necessary, the smaller size of the copper wire renders it more economical.
Apart from the economical working of the machine itself, whatever system may be adopted, it is of importance that cold once produced should not be wasted, and it is therefore necessary to use some form of insulation to protect the vessels in which liquids are being cooled, or the rooms of ships' holds in which the freezing or storage processes are being carried on.
This insulation generally consists of materials such as charcoal, silicate cotton, granulated cork, small pumice, hair-felt, sawdust, &c., held between layers of wood or brick, and forming a more or less heat-tight box.
The air spaces, two or three in number, are formed between two layers of tongued and grooved wood, and the total thickness of the insulation is about the same as when silicate cotton alone is used.
Of flake charcoal is generally sufficient for the insulation of the holds, though for deck-houses and other parts exposed to the sun the thickness must be greater.
In this condition there is complete electrical insulation between the jets, as may be proved by the inclusion in the circuit of a delicate galvanometer, and a low electro-motive force.
Given a certain allowable heat transmission, the principal points to be considered in connexion with insulation are, first cost, durability, weight and space occupied, the two last named being specially important factors on board ship. No exact rules can be laid down, as the conditions vary so greatly; and though experiments have been made to determine the actual heat conduction of various materials per unit of surface, thickness and temperature difference, the experience of actual practice is at present the only accepted guide.
This guard wire prevents any current which leaks over the surface of the insulator from passing through the galvanometer G, and the galvanometer indication is therefore only determined by the amount of current which passes through the insulator, or by its insulation-resistance.
Maxwell (Elementary Treatise, &c., p. 15) ingeniously applied this fact to the insulation of conductors.