(6) Applying experimental results to the expression given by J. J. Thomson for the loss of energy per centimetre (due to radiation) in passing through a medium containing ions, e and taking the negative corpuscles with = 7 x 106 and m e=10-20 as the sources of the radiation, the number of these corpuscles or electrons per c.c. for air under normal conditions is of the order 1022. (7) Quantitative results show that the secondary radiation. from metals, though of different penetrative power, is of the same nature as the primary X-radiation. (8) Results of the order of magnitude of those given above have been obtained with all metals experimented upon, though the secondary radiation from them differs considerably in character from the primary. University of Liverpool. LXII. The Thomson Effect in Alloys of Bismuth and Tin. By S. C. LAWS, B.A., B.Sc., St. John's College, Cambridge; 1851 Exhibition Scholar". I. Introductory. XPERIMENTS on some of the electrical properties of Bismuth-Tin alloys have already been made by Rollmann †, von Ettingshausen and Nernst ‡, Hutchins §, Spadavecchia, and Schulze T. The very interesting nature of the results obtained seemed to warrant a further study of the electrical behaviour of these alloys. The present communication contains an account of some experiments on the Thomson effect, or the amount of heat evolved or absorbed by a current in passing along a conductor in which a temperature-gradient is maintained, in such alloys. As is well known, this effect was first observed by Lord Kelvin **, who found that heat was evolved by a current passing down a temperature gradient in copper, whilst in iron an absorption of heat took place under similar circum stances. *Communicated by Prof. J. J. Thomson. + Rollmann, Pogg. Ann. lxxxiii. p. 78 (1851). Von Ettingshausen & Nernst, Wied. Ann. xxxiii. p. 477 (1888). Spadavecchia, Nuov. Cim. ix. p. 432 (1899). ¶ Schulze, Ann. der Phys. ix. p. 555 (1902). ** W. Thomson, Phil. Trans. cxlvi. p. 649 (1856). Lord Kelvin expressed the relation between the quantity of heat evolved or absorbed, the current and the temperature gradient in the form dQ=Cødf, where dQ is the amount of heat evolved or absorbed by the current in passing between two sections whose difference of temperature is de. The quantity & he called the specific heat of electricity. Comparative values of the specific heat of electricity for a considerable number of metals were obtained by Le Roux*, whilst absolute measurements have since been made by Haga †, Batelli and King §. In the experiments described. in this paper the method used by Haga has been adopted. Briefly, the method is as follows:-The temperature-gradient is obtained by immersing the ends of the experimental rods in baths at definite temperatures, and the value of the specific heat of electricity is found by comparing the change of temperature at a point when a current flowing along the temperature-gradient is reversed with the rise in temperature produced at the same point by a current in the bar when this is at a uniform temperature throughout. In this latter case the heat generated is known from Joule's law, so that, assuming that within the limits of the experiment change of temperature is proportional to heat developed, the amount of heat produced or absorbed in the former case is at once calculated. The Thomson effect in bismuth has already been investigated by Batelli, but for the sake of comparison, experiments were first made with bismuth and then with alloys containing increasing amounts of tin. II. Preparation of the Specimens. The bismuth used in the experiments was obtained as pure as possible by procuring the pure oxide and reducing this in porcelain crucibles with pure potassium cyanide. In this way, bismuth containing no impurity other than 0.02 per cent. of iron was obtained. The tin used was supplied as pure precipitated metal. The specimens with which the experiments were made were cast in the form of rods about 35 cms. in length, and as thin as their brittle nature would allow-that is, about 5 mm. in diameter. In the case of the alloys the moulds in which the rods were cast consisted of hard glass tubes, of the * Le Roux, Ann. de Chimie et de Phys. x. p. 258 (1867). + Haga, Ann. de l'école polyt. de Delft, i. p. 145 (1885); iii. p. 43 (1886). Batelli, Accad. delle Sci. di Torino, Atti, xxii. p. 548 (1887). King, Amer. Acad. Proc. xxxiii. p. 353 (1898). Batelli, loc. cit. Phil. Mag. S. 6. Vol. 7. No. 11. May 1904 2Q required length and diameter, sealed at one end; these were heated to about 240° C. in a bath of oil, and the molten metal, which had been previously kept liquid and well stirred for some hours, poured in and allowed to cool slowly. By cutting with a diamond and gently tapping, the glass was removed from the rods; these were then annealed by placing them in a wide glass tube and heating in an oil-bath to a temperature as near the melting-point as possible and very slowly cooling. Owing to the greater expansion of bismuth on solidifying, glass tubes could not here be used as moulds, for the metal, at the point where it first began to solidify, expanded to such an extent as to crack the tube, thus allowing any metal that still remained liquid to escape. A satisfactory mould which did not introduce impurities into the metal was made from slate. The mould was obtained in two pieces by clamping together two long slabs of slate, rectangular in cross-section, and drilling a hole of the required diameter along the length of the block with its axis coinciding with the line down the centre of the plane of junction. The walls of this cylindrical space were carefully polished with emery-powder, and the mould placed inside a cylinder around which a coil of german silver wire was wound. By sending a current of about 4 amperes through this coil, the mould could be heated to about 250° C. before the liquid metal was poured in. When the metal had solidified the two halves of the mould were separated and the rod withdrawn and annealed as before. III. Description of the Apparatus. To increase the magnitude of the effect to be measured and also to eliminate as far as possible errors due to the want of homogeneity of the material, two rods were used. These were placed parallel to one another at a distance apart of 12 cms., with one end of each passing through a rubber stopper into a bath in which water could be kept boiling, and the other end in a bath which might contain melting ice, or through which a current of cold water could be circulated. The ends in the hot baths were joined by a copper rod so that a current could be sent through the two rods in series. To the other ends of the rods leads were soldered, and the circuit completed through a battery of accumulators B, adjustable resistance R, ammeter A, and reversing key K1 (fig. 1). To measure the changes of temperature, thermocouples made from thin iron and nickel wires were used, one junction being fastened at a point on the one bar, the other at a point on the second bar, which, under the action of the temperaturegradient, assumed as nearly as possible the same temperature as the former. Fig. 1. Holols The junctions were separated from the rods by thin strips of mica, and both rods and baths closely packed with cottonwool. Usually four such thermocouples were placed at intervals along the rods, so that readings at different temperatures could be obtained without disturbing the packing round the bars. The free ends of the couples were joined up in series with one set of coiis of a sensitive differential Thomson galvanometer of low resistance, each couple being joined up to the galvanometer in turn by means of mercury cups. The galvanometer could be easily arranged to give a deflexion of 1 division of a scale reflected in a telescope for a current of 10-7 amperes, whilst a difference of temperature between the junctions of 1° C. produced a current of 2.2 x 10-5 amperes in the galvanometer circuit, so that a deflexion of 1 division corresponded to a change of temperature of degree C. To measure the current in the galvanometer a null method was employed, the current produced by the E.M.F. of the couple being compensated by adjusting a current in the other set of coils of the galvanometer. This compensating current was obtained from a single secondary cell joined up with a set of known resistances, and by means of a reversing key could be sent in either direction through the galvanometer. From a point R. (fig. 1) of the main circuit a current was taken off to pass through the galvanometer and high resistance Rs. The required magnitude of this compensating current was then obtained by first choosing a suitable value for R2 (100 ohms say), then adjusting R1 (5000 ohms), and finally R3 (1750 ohms); the current is calculated from the E.M.F. of the cell and known resistances. In order to decide accurately on the value of the compensating current necessary to annul the effect of the current in the junction circuit, it is necessary that the two circuits should be completed at the same instant. This was effected by placing in each circuit a key consisting of a vertical copper rod dropping into a mercury cup. The two rods, which were insulated from each other, were connected to the same movable upright, worked by a string over a pulley, and their lengths were adjusted so that they touched the surface of the mercury in the cups at the same instant. In this way the compensating current could be adjusted so that the galvanometer gave no deflexion when the upright was released and the circuit completed. This value of the compensating current is then proportional to the difference of temperature between the points of the rods to which the junctions are attached. IV. Practice of Method. The method of carrying out an experiment is then as follows:-The hot bath is filled with boiling water, which is kept boiling by means of a small gas-flame underneath; the cold bath is filled with melting ice, the ice being prevented from coming into contact with the rods by means of wire gauze caps surrounding their free ends. When the temperature of the rods has become steady throughout, a constant current-3 or 4 amperes-is passed |
