208 
MR. IT. L. CALLERDAR ON THE PRACTICAL 
tube of the thermometer. The tube DC is soldered through a biscuit tin, which is 
filled with water, renewed periodically, to keep the tube cool and the conditions 
uniform. Another brass tube with a cap at A slides over AD, so that the glass bulb 
is completely protected. The electrodes of the spiral go to mercury cups 1, 2, 3, 4, 
into any of which the conducting wires from the resistance-boxes may be dipped. 
R la or R 13 gives the resistance of the spiral and connecting wires. gives the resist¬ 
ance of the double electrodes, which is by symmetry twice that of the connecting wires. 
The resistance of the thick copper wires between the mercury cups and the boxes was 
measured. In Series IV. it was '0077 ohm, in Series V. '0164 ohm. The apparatus 
was heated in a very thick iron tube in a Fletcher tube furnace. The gas' could be 
adjusted by a screw tap with a graduated head, and might be used with an air blast 
for obtaining higher temperatures. 
For heating in steam a special pattern hypsometer was employed with a double 
jacket, through which a copious supply of steam was made to circulate from a boiler. 
The brass case AD of the thermometer should be made steam-tight, so that the 
steam cannot penetrate into the tube CD. 
Air thermometer No. 5 was an improvement on No. 3 in several respects. The volume 
of the bulb was four times as large (Y — 63'3 c.c.). A double electrode of fine wire 
was sealed through the glass, so that no part of the spiral itself was outside the bulb. 
The spiral was made of a different wire, diameter '013 cm., length 2 metres, 
resistance 20 ohms. It was therefore long and delicate. This proved a misfortune ; 
for some one, in inspecting the instrument in my absence, after it was made, held it in 
a vertical position and deformed the platinum spiral completely, so that it was not 
uniformly distributed throughout the length of the bulb. This could not be cured 
without making the whole instrument afresh, and was probably a serious source of 
error whenever the temperature was at all unsteady (see Observations (11), (13), 
Series Y.). The observations were taken precisely as before, perhaps with greater 
care and skill, acquired by experience. The electrode which passed out through the 
capillary tube consisted of 6 in. of Pt wire, chain. '036 cm., fused on to 12 in. of Cu 
wire, No. 26 B.W.G., dipping into mercury cup 1 (fig. 7). It was of necessity single. 
A double electrode was made of the same wires, exactly to imitate it, and coated with 
the same hard glass for insulation, and placed in close proximity to the capillary tube, 
inside the small brass tube CD (fig. 7) which carried the thermometer. Its position 
must be carefully adjusted, for any want of symmetry or misplacement of the imitation 
electrode may introduce apparent errors in the resistance measurements. The other 
double electrode consisted of a short length (10 cm.) of fine Pt wire, to the middle 
point of which the end of the spiral was fused : the fine wire was bent double and 
fused through the glass at A (fig. 7), and its two ends projecting were fused on to 
thicker ('096 cm.) Pt wires, coated with hard glass for insulation, and extending as 
far as the Cu-Pt junction of the other electrode inside the brass tube CD (fig. 7), 
where they were fused on to No. 20 Cu wire leading to the mercury cups 2, 3. By 
