54 ELECTRICITY [HEATING EFFECTS, tlve dig- charge. On the Passage of Electricity through Insulators. Hitherto we have divided bodies into conductors, through electricity passes under the influence of any electro- j however small, and non-conductors or insulators, through which electricity will not pass, no matter how great the urging force. In practice, however, when the value of the electromotive force reaches a certain limit, electricity does pass through a non-conductor. A discharge of elec tricity taking place suddenly in this way through a non conductor is called a " disruptive discharge" The power of a non-conductor to resist up to a certain limit the passage of electricity through it has been called its dielectric strength. The dielectric strength of any medium is greater the greater the electromotive force it will stand, when placed say between two parallel metal plates arranged in a given way, before it is broken through by the disruptive discharge. We shall by and by attach a definite quanti tative signification to the term, but the general notion will be sufficient for the present. i Although it may be found when both phenomena have f been more fully analysed, that conductive and disruptive discharge are really two different aspects of one and the same phenomenon, yet for the experimenter they are two distinct things, which must not be confounded. This would be the place to set forth the quantitative rela tions which regulate the electromotive force required to pro duce disruptive discharge, the quantity of electricity that passes under given circumstances, and the dielectric strength of different media; in fact, to lay down for disruptive dis charge a law corresponding to the law of Ohm for metallic and electrolytic conduction. The present state of electrical science, however, does not permit us to do this in a satis factory manner. Experiment has not as yet led to a single dominant principle, like Ohm s law, which will account for all the phenomena of disruptive discharge. The best theory of the subject is Faraday s, which will be gone into under "disruptive discharge in gases." Observation and experi ment, on the other hand, have been occupied for the most part with the various transformations of energy which ac company the disruptive discharge. We prefer, therefore, to discuss the whole matter under the single head "disrup tive discharge." TRANSFORMATIONS OF ENERGY ACCOMPANYING THE ELECTRIC CURRENT. Under this head we propose to discuss (to use a word of Rankine s) the energetics of electricity. It may be objected that this heading might have been put over a good deal of what has gone before, and we shall, for con venience, treat certain matters under it which, in a strictly logical division, would have found a place elsewhere. If we had formed a definite conception of what we call elec tricity had, for instance, assumed that it is a material fluid, having inertia like other fluids, then no doubt the energetics of the subject could have been much extended. As it is, we think that advantage is to be gained by associating in our minds the experimental laws which we are now to arrange under the above heading. We shall consider (1) the heat developed in metallic and electrolytic conduction, and at the junctions in heteroge neous circuits ; (2) the mechanical, sound, heat, and parti cularly light effects accompanying disruptive discharge; (3) the energy of magnetized iron and steel, and of electric currents in the neighbourhood of the electric current (electro- magnetism) ; (4) the energy of the electrotonic state, or electrokinetic energy (magneto-electric induction). In this list ought to be included the potential energy of chemical separation, which would come under the head of electrolysis. At present, however, electrolysis is quite as much a chemi f at cal as an electrical subject, arid it has been found convenient to treat it in a separate article (see ELECTROLYSIS). Some points in connection with it have already been touched upon, and a few more will come up in (5), which treats of sources of electromotive force, and deals with the ques tion, whence comes the energy which is evolved in the voltaic circuit 1 a question the answer to which is for the most part experimental and practical the only one, in fact, that the state of electrical science permits us to give. Heating Effects. It is easy to show, by a variety of simple experiments, Devel that a current of electricity heats a conductor through which it passes. In the case of moderately strong currents the heat developed is perceptible to the touch; the wire may, in the case of very strong currents, be raised to a white heat ; it may melt, and even be volatilized. In the case of very weak currents, the heating effect may be de monstrated by passing the current through the spiral of a delicate Breguet s thermometer. We find, when we examine the experimental data on the subject, that heating effects may be conveniently divided into two distinct classes. In the first of these the fundamental law is that the de velopment of heat in any part of a linear circuit varies as the resistance of that part multiplied by the square of the current. In the second class the development of heat varies as the first power of the current. The heating effects of the first class are obviously independent of the direction of the current, and are irreversible ; and the more we examine them the more they appear to correspond to the loss of energy by the frictional generation of heat in ordin ary machines. In the language of the dynamical theory of heat, the part of the energy of the electric current which disappears in this way is said to be dissipated. The effects of the second class change their sign when the direction of the current is changed ; so that, if anywhere there was evolution of heat when the current flows in one direction, then, when the current is reversed, there will be absorption of heat to an equal extent. We shall find that we have great reason to believe that such effects are strictly reversible. 1 In order to get a satisfactory founda tion for the simple theoretical views which we have thus indicated, it is essential to be able to separate the two classes of effects. Now, this is possible to a very great extent even in practice. The effects of the first class in crease much more rapidly with the strength of the current than those of the second, so that, by sufficiently increasing the current, we can make the effects of the second class as small a fraction of the whole heating effect as we please; while, on the other hand, by sufficiently decreasing the current, the preponderance of the second class may Le increased to any desired extent. We shall in what follows suppose the two classes of effects separated in this way. Discliarge of Statical Electricity. One of the earliest attempts to study the heating effects of the electric dis- charge was made by Kinnersley. He constructed an thermoelectrometer, which consisted of a closed glass vessel, in which were fixed two metal balls communicating with electrodes outside the vessel. The bottom of the vessel was filled with a little coloured fluid, which com municated with a tube having a vertical arm rising outside the vessel. W 7 hen a spark passed between the balls, the heat developed caused the air to expand and force the liquid into the vertical tube, the rise of level in which indi cated the degree of expansion, and, by inference, the amount of heat developed in the spark. Sir Wm. Snow Harris 2 revived this instrument of Kin- nersley s, and improved it by stretching a fine wire between 1 That is, in the thermodynamic sense. 2 Phil. Trans., 1827. f ^j
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