by means of the galvanometer, to have risen to the desired point, the flame was withdrawn, and the cold water, at the temperature of the laboratory, applied as before. It was also found advisable for steadiness of temperature to stop the current of cold water through the water-jacket, and to warm up the water in the water-jacket (which, while the flow continued, was kept at the temperature of the public supply) to be roughly the same as the air temperature of the laboratory.
The Sprengel pump used for exhausting the enclosure and the duplicated MacLeod gauge used for determining the residual pressure are fully described, and certain difficulties and uncertainties are pointed out, in my former paper,[1] and need not be further spoken of here.
For the purpose of calculation two pieces of numerical information are still required. One is the capacity for heat of the copper globes; and the other, the cooling surface.
The capacity for heat was determined very carefully for these particular globes by Mr. D. Macfarlane in 1872; and my determination has not sensibly varied from his result. I propose, however, to make a fresh determination of this constant as soon as possible.
The surface of the globe is obtained by careful measurement with calipers. But it will be observed that, besides loss of heat by radiation, there is also a loss by conduction along the two thermo-electric wires. When a sooted globe is cooling in full air pressure the amount of heat conducted along the thin wires used is very small in comparison with that lost by radiation and convection. But when the globe is cooling in vacuum or, at any rate, when a highly polished silvered globe is cooling in vacuum, the heat lost by the wires is by no means negligible in calculating the value of the surface emissivity.
In making use of the results obtained up to the present time I have used for calculation an equation of the form
ing gave rise to a most disturbing difficulty at the commencement of the corresponding vacuum series; because it was desirable to commence the observations on cooling as soon as the cooling commenced, but it was not till ten or fifteen minutes later that a proper vacuum could be secured, even with the most vigorous use of the pump.
- ↑ Phil. Trans.,' 1887, pp. 444–446. There is just one remark which it seems worth while to make, on account of repeated misunderstanding, namely, that the MacLeod gauge does not measure (nor profess to measure) pressure in the pump of any gas collapsible by pressure, for any reason whatever, at the existing temperature of the gauge. Thus it does not measure the pressure of mercury vapour which may be present in the so-called vacuum, nor the pressure of vapour of water, for example. Nor does it even measure the pressure of any gas which may with the help of a small pressure be caused to collapse against the walls of the tube, or by absorption into, or on, the mercury used in the gauge. I have shown the possibility of this last-named occurrence in a paper on an "Improved Mercury Pump," British Association, 1886.
