As A_1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X, it is put in connection with X, and has a new charge, this time negative, driven into it by induction from B_2. Positive electricity, then, being carried by the conducting patches from right to left on the upper half of the A plate, and negative from left to right on its lower half. A similar process is taking place on the B plate, but in this case the negative electricity is going from left to right above, and the positive from right to left below. On the whole, therefore, positive electricity is being supplied to the left hand main conductor Z by both upper and lower plates, and negative to Z_1. [3] NOTE.--The discovery of this property of matter is due to Stephen Gray, who, in 1729, found that a cork, inserted into the end of a rubbed glass tube, and even a rod of wood stuck into the cork, possessed the power of attracting light bodies. He found, similarly, that metallic wire and pack thread conducted electricity, while silk did not. Gray even succeeded in transmitting a charge of electricity through a hempen thread over 700 feet long, suspended on silken loops. A little later, Du Fay succeeded in sending electricity to no less a distance than 1,256 feet through a moistened thread, thus proving the conducting power of moisture. From that time the classification of bodies into conductors and insulators has been observed. [4] NOTE.--Copper is pre-eminently the metal used for electric conduction, being among the best conductors, it is excelled by one or more of the other metals, but no other approaches it in the average of all qualities. [5] NOTE.--A current of electricity always flows in a conducting circuit when its ends are kept at different potentials, in the same way that a current of water flows in a pipe when a certain pressure is supplied. The same electrical pressure does not, however, always produce a current of electricity of the same strength, nor does a certain pressure of water always produce a current of water of the same volume or quantity. In both cases the strength or volume of the currents is dependent not only upon the pressure applied, but also upon the _resistance_ which the conducting circuit offers to the flow in the case of electricity, and on the friction (which may be expressed as resistance) which the pipe offers to the flow in the case of water. [6] NOTE.--The prefixes “meg” and “micro” denote million and millionth. For example, a megohm equals 1,000,000 ohms, a microhm equals 1/1,000,000 of an ohm. [7] NOTE.--The reciprocal of a number is equal to 1 ÷ the number; for instance, the reciprocal of 3/20 = 1 ÷ 3/20 = 20/3 = 6-2/3