436 
MR. J. T. BOTTOMLEY OX THERMAL 
massive copper-jacket for the purpose, when Sir William Thomson proposed to me 
to use a cylindrical jacket made up of a large number of thin co-axal copper-tubes, 
and pointed out to me the great advantages possessed by such a composite jacket in 
equalising the temperature. Accordingly, the heating jacket shown in fig. 4 , b, is made 
up of eight co-axal copper tubes, each with a copper bottom, fitting closely one inside 
another, a few turns of the finest asbestos yarn being wrapped spirally round each of 
the tubes before it is pushed into the tube in which it fits. The internal diameter of 
the smallest tube is 3 ^ centims., the outside diameter of the outermost tube is 6 centims., 
the length over all is 33 centims. The sheet copper used for making the tubes is 
| mm. in thickness, and the weight of the whole is 4 kilos. I have used electrolytic 
copper, technically known as “conductivity copper,” supposing it probable (though I 
have not had time to verify the supposition) that the pure copper, which shows such 
marked superiority to less pure copper as to electric conductivity, may also possess at 
least some superiority as to thermal conductivity. 
The copper-jacket is closed with a stopper of woven asbestos about 5 centims. long. 
Holes are pierced in the stopper, through which pass two thick copper electrodes and 
the stem of an air-thermometer, to be described presently. The jacket is heated 
sometimes by two rows of six Bunsens in each row; the Bunsen tubes in each row 
being screwed into holes tapped in a -|-inch iron pipe. I find the most convenient 
heater, however, to be one of Fletcher’s large “ solid flame ” burners. This burner 
gives an enormous flame, enveloping the whole of the copper-jacket (which is placed 
inclined as shown in fig. 4 , a), and easily raising it to a good red heat through and 
through. The electrical measuring apparatus which I use is amply sensitive to a 
change of two or three degrees of temperature ; and, using the jacket and heater as 
just described, it is easy to maintain the internal cavity at a good red heat without 
any such variation of temperature for ten minutes or twenty minutes at a time, or 
longer if need be. 
The air-thermometer used is shown separately in fig. 4 , c. It is made from com¬ 
bustion tubing which remains perfectly hard and unyielding at a moderate red heat. 
The thermometer is formed from tubing, half-an-inch in internal diameter, by drawing 
off a portion ab, about 2f inches in length, and then drawing out the extremities so as 
to form capillary tubes ad and be. One of these, be, is for convenience hent up 
nearly close along the bulb of the thermometer. The other capillary tube is sufficiently 
long to pass through the asbestos stopper of the heating jacket, and to project down 
from it so far as to bring the bulb to the middle of the heating jacket. 
The thermometer, as soon as it is drawn oft) is attached, with both ends c and d open, 
to a water aspirator, and a current of air, purified and dried, is drawn through it. 
While the current of air is passing, the whole of the glass is raised to a good red heat 
two or three times by means of a powerful Bunsen flame. In this way every trace of 
moisture and of condensed gas is driven off from the walls of the tube ; and it appears 
