Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/65

Rh HEATING EFFECTS.] ELECTRICITY 55 ieri- the terminals inside the vessel, so that the heat measured its of was now ti^ evolved in a metallic conductor. With this improved instrument he made a number of valuable ex periments on the heating of wires by the discharge of a Leyden battery, whose charge was measured by a Lane s electrometer. As suming that the heat developed varies inversely as the conductivity of the wire (which is not the case), he arranged the metals in a series which agrees with that given later by Riess, although the numbers given do not properly represent the conductivities owing to the erroneous assumption on which they are deduced. Harris observed that the specific conductivity of alloys is often less than that of either metal, and that a very small admixture of another metal con siderably reduces the conductivity of pure copper. He also arrived at the result that the amount of heat developed in a wire varies as the quantity of electricity which passes in the discharge, but seems to have concluded that the amount of battery surface used had no effect. 1 5S. Riess made two very important improvements on the rnio- thermoelectrometer by substituting spirals for the straight tro- w j re O f H arr i Sj an( j by inclining the tube containing the e1 liquid so as to be nearly horizontal. The sensibility of the instrument was thus greatly increased. Riess took up the whole question of the heating of wires, and investigated it thoroughly. The actual instrument which he used is represented in figure 27 (taken from his ReibungsdcctricitiU). It consists of a glass tube of Fig. 27. narrow bore, 16 to 17 inches long, to which is blown a glass globe b to 4 inches in diameter. This tube is partially filled with some coloured iluid which confines the air in the globe, a wide reservoir at the other end of the tube allows the fluid to accumulate without sensible change of level. The stand of the instrument consists of two pieces hinged together, so that the tiibe can be placed at a small in clination to the horizon. The rest of the instrument will be under stood from the figure. Details concerning the manipulation will be found in the Eeibungselcctridtdt, Bd. i. 410. When the fine wire is heated by a current of electricity, the heat developed is divided between the wire and the air ; the expansion very quickly reaches a maximum, and the level of the liquid in the fine tube becomes stationary for a moment. If in be the number of scale divisions between its original and final positions, we have (see Kiess, I.e., or Mascart, t. i. 325) = 7nA and H = (1), where T denotes the amount by which the temperature of the wire would have risen had no heat left it, and H the whole amount of heat developed by the current. C and W are the mean specific heat and weight of the wire, and A and B constants, which depend on the make of the instrument, and on the initial temperature and pressure of the air. A very convenient form of thermoelectrometer, called the thermometre inscripteur, has been used by Mascart (I.e.). 1 Phil. Trans., 1834. The alterations of pressure are registered automatically on a revolving drum, after the manner of the pulse-registering instrument of Marey. One advantage of this instrument is that it gives a representation of the course of the tem perature in the apparatus. In most of his experiments Riess used batteries of Leyden jars. The jars were all as nearly as possible alike, and the inner armatures were in general connected together. The quantity of electricity given to the battery was measured by means of a Lane s jar, the balls of which were placed at a distance of about a line apart. The battery was then dis charged through the thermo-electrometer along with any external circuit connected with it. It is of great importance in such experiments as we are General now describing to examine what happens at the place where consider the circuit is closed. This closure is effected by bringing atlon8- two metallic balls into contact. But before contact is reached, a spark passes in which sound, light, and heat are given forth, in a word, energy evolved. When the resis tance of the circuit is small, this spark passes at a consider able distance, and is very intense, no matter how quickly the conductors are brought together. The energy consumed in this case is a considerable fraction of the whole energy given out by the discharge. If, however, the resistance of the circuit through which the discharge takes place be con siderable, the electricity takes longer to accumulate suffi ciently to raise the electromotive force between the balls to the discharging limit. We may, therefore, by operating quickly, get the balls very nearly in contact before the spark passes. In this case the spark is much less intense, and the fraction of the whole energy which appears in it is very small. Riess made some very valuable experiments on this point. He arranged an air-break in the circuit of the thermoelectrometer, which he could widen or narrow at plea-sure, and discharged his batteries through this circuit in the usual way. He found that as the gap is widened the amount of heating in the thermometer is at first increased, but after a certain length of break is attained it decreases again. It must be remembered that we have now two air breaks in our circuit of discharges, the discharging break and the inserted break. One effect of the inserted break is to diminish the intensity of the spark at the discharging break, and cause a decrease of the energy which appears there. On the other hand it makes the discharge of the battery incomplete, so that part of the potential energy is not exhausted. It is very likely to the opposition of these two effects that -the peculiarity observed by Riess is due. Mascart has observed a similar phenomenon in disruptive discharge through oil of turpentine. At all events Riess showed that, when the inserted break was not longer than ^ths of a line, the heating in the thermometer was the same as when there was no break at all. Hence, if we make the resistance of our circuit so great that the spark at the discharger is not longer than j^ths of a line, the energy consumed there may be neglected. The resistance of the connections belonging to the battery and the thermometer were always very small com pared with that of the thermometer wire, and the wire, if any, inserted outside the thermometer ; so that, if the resis tances of these be R and S, the resistance of the whole circuit may be taken to be R + S. The law to which the General experiments of Riess led can be expressed by means of the l av - formula S. H = K + S v (2), where Q is the amount of electric potential energy which has disappeared, and H the amount of heat (measured by its dynamical equivalent) developed in the wire of the thermometer, whose resistance is S. In the case of the complete discharge of a battery of n