Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/68

Rh 58 E L E C T K I C 1 T Y [HEATING EFFECTS. The first term is Joule s, the second Peltier s effect. Here the coefficient of the Peltier effect appears as an electromotive force. AVe shall return to this again. Glowing, Glowing, Melting, Volatilization, &c. If a wire lost melting, none O f the heat generated in it, then, for the same wires current, the rise in its temperature during a given time would vary as its specific resistance directly, and as the product of its specific heat and density into the fourth power of its diameter inversely. Thus, T, r, c, p, d denoting these quantities in the order named above, If we have a given battery of electromotive force E, and a circuit connected with it of resistance R, and we insert a wire of length I specified in other respects as above, the current will be 4:lr where s== If the diameter of the wire be given, then Soc / - } and Tec - 2 , which is a maximum when R = S, that is, when the length of the wire is such that its resistance is equal to that of the rest of the circuit. Owing to our ignorance of the exact law of cooling, and of the manner in which the resistance and specific heat of most metals change at very high temperatures, it is very difficult to predict beforehand to what temperature a given current will raise a given wire. It is, as may be supposed, still more difficult to predict the effect of a given discharge from a Leyden battery. According to Riess, the pheno menon of glow in this case is complicated by concomitant effects of specific nature. 1 If we assume Newton s law of cooling, i.e., that the heat given out is proportional to the surface of the wire and to the elevation T of its temperature over that of the surrounding medium, then, I denoting the strength of the constant current which heats the wire, we have, when a constant temperature has been attained, I 2 = const. x Td 3, for wires of same length and material but differ ent diameters. If we compare the apparent brightness of the wires, by causing them to illuminate a screen at a constant distance off, and assume that the light given out is proportional to Td, then, if two wires of diameters d i and d. 2 have the same apparent brightness, T 1 tZ 1 = T 2 c? 2, and I 1 -s-d l = I 2 -:-rf 2. In other words, the strength of current requisite to bring a wire of given length and material to a given brightness of glow varies directly as its diameter. A law of this nature is, of course, merely a rough approximation; Miiller and Zollner, however, have made experiments which agree with it within certain limits. The method of Zollner is interesting (see Wiedemann s Galvanismus). The temperature of a glowing wire is very sensitive to external circumstances, such as air currents, &c. These effects may be very strikingly shown by balancing the wire in a Wheatstone s bridge against a resistance of thick wire, a strong current being sent through the bridge. The behaviour of the wire in different gases is very remarkable. If a wire which is glowing in air be suddenly immersed in a jar of hydrogen or coal gas, the brightness will be very much reduced, in fact, in most cases the glow will entirely disappear. 2 This is owing to the greater cooling power of hydrogen, of which evidence is furnished by the experiments of Dulong and Petit. 3 The cooling power of different gases was shown by Grove. He arranged a platinum wire in a glass tube, which could be filled with different gases. The current of the same battery was sent through the wire and through a voltameter. When the tube was filled with hydrogen or olefiant gas, the amount of gas evolved in the voltameter per minute was 77 and 7 &quot;0 cubic inches respectively. The numbers for the other gases experimented on varied from 6 6 to 61. They stood in the following order: CO, CO 2, 0, air (2 atmos.), N, air (1 atmos.), air (rarefied), Cl. Experiments of a similar nature were made on liquids. Clausius carried out a calculation of the cooling effect of different gases, and found that the experimental results could be satisfactorily accounted for. 4 When the strength of the current is sufficiently increased, the wire ultimately fuses, or even volatilizes. The pheno menon is in general complicated. In air, for instance, the 1 Reibungselectricitiit, Bd. ii. 557 sqq. 2 Grove, Phil. Mag., 1845, or Wied. Galv., Bd. i. 679. 3 Poggendorff, Pogg. Ann., Ixxi., 1847. 4 Wied. Galv. (I. c.), or Pogg. Ann., Isxxvii., 1852. wire burns, and the oxidization once started may take a greater share in raising the temperature than the current does, so that the destruction of the wire may take place under certain circumstances with a current, which, under other conditions, would scarcely make it glow. When dis charges from a Leyden battery are used it is very difficult, if not altogether impossible, to get melting unaccompanied with mechanical disaggregation of the wire. The reader who wishes for further information concerning these matters, will find the sources sufficiently indicated in Wiedemann, Riess, and Mascart. This department of electricity is very fruitful in I np popular lecture-room experiments. We shall quote one or ex P ( two of these, and refer the reader to popular treatises for mo11 more of the same kind. On a sheet of thin card-board is pricked a design, generally what is understood to be a portrait of Franklin, two pieces of tinfoil are pasted on the ends of the card by way of electrodes, and between these a piece of gold leaf is laid. On the other side of the card is placed a piece of white paper or silk. The whole is then tightly screwed up between two boards. When an electric discharge is sent through the gold leaf it volatilizes, sending the disintegrated particles through the holes in the card-board. In this way an im pression of the portrait is obtained. If a current be caused to heat a pretty long thin platinum wire to dull redness, and a portion of the wire be cooled by applying a piece of ice to it, the remainder of the wire will glow much more brightly than before ; whereas, if a portion be heated by a spirit-lamp, the reverse effect takes place. The reason is that the current is strengthened in the one case by the decrease of the resistance in the cooled part, and weakened in the other by the increase of resistance where the wire is heated. When two curved metal surfaces rest upon each other, a current passing from the one to the other encounters considerable resistance at the small area of contact. The heat developed in consequence of this causes the parts in the neighbourhood to expand very quickly when the contact is made. This very often gives rise to rapid vibra tory movements in the conductors. The Trevelyan rocker 5 can be worked in this way (see art. HEAT), bells rung, &c. The best known experiment of the kind is Gore s railway. This consists of two concentric copper hoops, whose edges are worked very truly into the same plane. A light copper ball is placed on the rails thus formed, a current from two or three Groves is sent from one hoop to the other, and the ball set in motion. If the ball be very true, and the railway be well levelled, the energy supplied by the swelling at the continually changing point of contact is sufficient to keep up the motion, and the ball runs round and round, emitting a crackling sound as it goes. 6 The Voltaic Arc. When two electrodes of volatile or Elec readily disintegrable material forming the poles of a power- J g 11 ful battery (say 30 or 40 Grove s cells) are brought into contact and then separated, the current continues to pass across the interval, provided it is not too great. The con ducting medium appears to be a continuous supply of heated matter, suspended in glowing gas or vapour. This phenomenon seems to be more akin to the subject we are now discussing than to the disruptive discharge of which we shall speak by-and-by. The light thus generated with a large battery, especially when electrodes of graphitic carbon are used, is brilliant in the extreme. It was thus that Davy first obtained the phenomenon. 7 With a battery of 2000 cells he obtained a luminous arc 4 inches in length, and when the carbons were placed in an exhausted receiver the arc could be lengthened to 7 inches. The fact that the electrodes must be brought in contact in order to start the light is quite in accordance with what we know of the extremely small striking distance of even very powerful batteries. When the contact is made, the place where the electrodes touch, owing to itssmall section, is intensely heated; the matter begins to volatilize, and then the current is kept up by the quickly increasing clowd of metallic 6 Wied. Galv., Bd. i. 726. 6 This motion has been attributed to electromagnetic action. Such an explanation is quite inadmissible. 7 Phil. Trans., 1821. According to Quetelet, Curtet observed the light between carbon points in 1802. Wied. Galv., T5d. i. 783.