DISTRIBUTION.] ELECTRICITY 19 suspended by a white silk string, we shall find that the electro meter needle is deflected through a certain angle, the spot of light going a certain dis tance to the right, say, of the scale. It will be found that, provided the ball C is more than a certain depth (about 3 in. in Farady s ex periment) btilow the mouth of tliw pail, no further motion of the ball, right or left, up or down, will affect the indications of the elec trometer. It will also be found that the same indications will be got to whatever point of the outside of the paiJ the electrometer wire p- Earth is attached. If we dimmish or increase the + electrification of C, the electrometer deflection will diminish or increase accordingly. If we introduce a negatively electrified ball C , the deflection will be to the left, and everything else as before. If C gives a certain positive (right) deflection, and C an equal (left) deflection, then if we introduce C and C together, the deflection will be zero. If C and C be both + electrified and give equal + deflections, then introduced together they will give a double + deflection, and if three such balls, all giving equal + deflections, be introduced together, they will give a treble + deflection. It is obvious that this experiment of Faraday s not only gives a very ready test of the electrical state of bodies, but at once suggests the notion of electrical quantity, and a theoretically possible electrostatic unit. Suppose, in fact, we take for our test the deflection of a Thomson s electro meter of given sensibility, then we might specify as a unit of electrical quantity the quantity of + electricity on or in a brass ball of given size, which will produce with a given cage a certain given deflection of the electrometer. To make this definition useful we must have the means of transferring a given charge from one body to another, and charging a body with any multiple or submultiple of our unit, and of charging a body with any multiple or sub- multiple of the unit of negative electricity, which we may define as the quantity of - electricity which will just annul the action of the unit of + electricity in the electric cage. All these requirements may be satisfied by suitably mo difying Faraday s experiment. We saw that we might move the ball about in the middle of our electric cage without affecting the electrometer de flection ; we find, moreover, that when we withdraw the electrified ball without touching the cage, the needle returns to zero. If, however, before withdrawing the ball we cause it to touch the inside of the cage, the electrometer deflec tion remains the same as before, and after the ball has been removed the deflection is still the same, while if we examine the ball, we find that all traces of electrification have disap peared. 3- To transfer a given quantity of electricity. If we pro vide ourselves with two cages, a large one G, and a small one H, and take a ball C, electrified positively with unit quantity as above defined, then testing C in cage G, in connection with the electrometer, we get a certain de flection D. If now we transfer the electrification of C to H, by the process just described, and then put H inside G, we shall get the same deflection D as before. It appears, therefore, that we can transfer electrification from one body to another without loss ; we thus fulfil one of our require ments, and give an additional justification of the use of the word quantity in the present case. e To yet any multiple or submultiple of the electric unit. We may repeat the process above performed on the small e cage H by touching its inside with the ball C, again electri fied to unit quantity. All tb.3 electrification will pass to II Fig. 6. We can thus as before, and if we now test E in G we shall get a deflec tion 2 D. We can in this way get any multiple we please of the unit charge. If we take the elec trified brass ball C and touch it by a per fectly equal neutral ball C , on introducing C into G we shall get deflection J D; if we touch C again by C , previously rendered neutral, we shall get deflection ^ D, and so on ; if we had touched C simultaneously, as in fig. 6, with two equal neutral balls, we should have got deflection D, and so on. get any submultiple of our unit charge, To get a given multiple and submultiple of the negative unit. This is possible when we can get a quantity of electricity, which will just destroy the action of a given quantity of + electricity in the electric cage. If we intro duce our given quantity of + electricity into the cage H, without allowing the conductor carrying it to touch the cage and at the same time put the outside of the cage in communication with the ground, then if we remove the conductor with the given quantity of + electricity and put it in G, it will give the same + deflection as before, while H tested in the same way will give a negative deflec tion exactly equal to the former, and if both be introduced together there will be no deflection. We can, therefore, in this way get a quantity equal and opposite to a given + quantity. 1 Electrical Distribution. Experiments had been made before Coulomb s time to determine what effect the nature of a body has on electri cal distribution. Gray and White concluded, from an experiment with two cubes of oak, one hollow and the other solid, " that it was the surface of the cubes only which attracted." Le Monnier 2 showed that a sheet of lead gave a better spark when extended than when rolled together. These experiments point to the conclusion that electrical distribution in conducting bodies depends merely on the shape of the bounding surface. We may make experiments confirmatory of this conclu sion with the electric cage. If we electrify a brass sphere A, and then touch it with another sphere B, and test the electrification of B in the cage, we shall find that the amount of electricity taken by B is always the same, what ever its material may be, so long as the radius of its exter nal surface is the same. Experiment is unable to detect any difference in this respect between a solid sphere of lead and the thinnest soap-bubble of the same radius. Coulomb took a large cylinder of wood, in which he made several holes four lines in diameter and four lines deep. Having electrified the cylinder and insulated it, he examined its electrical condition by means of the proof-plane. This instrument, so much used by Coulomb, consisted merely of a small disc of gilt paper (in this case a line and a half in diameter) fastened to the end of a needle of shellac. The disc is applied to any point of a body whose electrifi cation we wish to test so as to be in the tangent plane to the surface of the body. Assuming for a moment, what we shall by-and-by prove, that electricity resides on the surface of bodies, it is natural to suppose that the proof- plane, when placed as described, will form part of the bounding surface, and will therefore take up as much elec tricity as was originally on the part of the surface which it 1 The substance of the above and a good deal of what follows is taken from Maxwell s Electricity and Magnetism, vol. i. We recommend the student to read his remarks on quantity, 35, venturing to suggest, as an illustration of the transmission of energy by action at a distance, the case of two bar magnets, in which the energy of vibration is trans mitted and retransmitted periodically. See Tart s tiecent Advances in. Physical Science, p. 179.
8 Mascart t. i. p. 90.