PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 693 
66. As it was, the result afforded a most striking and immediate confirmation of 
the conclusion previously arrived at regarding the electrical convection of heat in 
iron. Every time the current was reversed, the liquid fell rapidly (showing a rise of 
temperature) in the thermometer next the end, by which the current nominally 
entered, and rose rapidly (showing a fall of temperature) in the other. 
Mr. Joule assisted in this experiment, and was satisfied with the evidence it afforded 
in favour of the conclusion that the Resinous Electricity carries heat with it in iron. 
6/. Unsuccessful attempts were next made with tubular conductors of different 
metals; and in endeavouring to get decisive results regarding the qualities of copper 
and brass, I again had recourse to the form of conductor used in the preceding ex- 
periment. The new conductors were, however, made of much thinner sheet metal 
than those of the iron, to admit of a less powerful battery being used ; and conse- 
quently, in each case, a frame-work had to be arranged to hold the conductor steady. 
Great difficulties were met with in continually repeated failures of the air-thermo- 
meters. It was therefore found necessary to have metallic tubes continued down- 
wards several inches from the bulbs, so as to prevent the wax by which the glass was 
cemented from being melted by the heat. The battery, however, had also to be 
reduced to a single zinc plate in one of the wooden cells, as with more of the bat- 
tery than this, the heating action had been found to be so sudden in the thin copper 
and brass conductors, as almost immediately to melt the solder about some of the 
bulbs, and so make one or more of the thermometers fail before the regulating 
action of the break was applied. Notwithstanding all precautions, the central thermo- 
meter failed in each case, and the action of the lateral thermometers was very unsatis- 
factory both in the copper and in the brass conductor. Tbe central thermometer 
could, however, be well dispensed with by regulating by the break one or other of the 
lateral thermometers ; and thus, after many unsuccessful attempts, experiments were 
made on copper and brass conductors, which, although still unsatisfactory, showed 
decidedly the looked-for convective effect. In each case, the thermometer which was 
not kept to one point by the regulator, always showed an increase of temperature, 
both in the copper and in the brass conductor, when the current was reversed so as 
to enter by the end remote from it, and showed a diminution of temperature when 
the current was again reversed so as to enter by the end next it. Hence it appeared 
that the Vitreous Electricity carried heat with it in both copper and brass. 
68. The lateral metallic tubes branching down from the conductor to carry the 
glass tubes of the air-thermometer, constituted a great defect in the plan of apparatus 
used in the experiments just described ; and the only way of avoiding it appearing 
to be to make the glass tubes pass through the body of the conductor itself, so as to 
admit of their being cemented air-tight at its cool ends, I again had recourse to the 
tubular form of conductor which had been tried unsuccessfully before. 
69. A tube made of very thin sheet platinum, soldered with gold, was arranged in 
the following manner : — A glass rod, inehes long, wrapped closely round with thin 
4 Y 2 
