RADIATION IN ABSOLUTE MEASURE AT VERY LOW TEMPERATURES. 
353 
1 centim. long and 0'4 centim. in diameter. One of these cylinders is screwed into 
the centre of the copper globe, and the other, carrying the outside thermoj unction, is 
tied to the bulb of a pentane thermometer p, and is placed in the bath of liquid air 
which surrounds the enclosure.* 
9. The cooling globe g is suspended by the wires of the thermojunction. Its weight 
is borne by the platinoid wire, which passes through a line hole drilled in the wall of 
the brass tube, and the wire is guided to hang centrally in the tube by the support s. 
After the length of the platinoid wire has been adjusted so that the copper globe shall 
hang at the centre of the spherical enclosure, the wire is soldered at the point r, 
where it passes through the wall of the tube. The platinum wire passes out through 
a sealed joint in the roof of the glass tube tt, and to insulate it from the platinoid 
wire, and from the brass tube in which the two hang side by side, the platinum wire 
is enclosed in a line glass tube or sleeve, v, which completely -protects it. 
10. The ends of the thermojunction circuit are brought to a mercury reversing key, 
which has four cups. To the copper electrodes of one pair of cups the thermo¬ 
junction wires are soldered ; and the electrodes of the other pair of cups are joined 
up, through an Ayrton Universal shunt, and a 100,000 ohms resistance, to a standard 
Clark cell. Two amalgamated copper rods, the terminals of the galvanometer circuit, 
can be revolved so as to make contact with either pair of mercury cups. This allows 
the constant of the galvanometer, and the thermojunction value, to be checked with 
reference to the value of the standard Clark cell whilst the experiment is proceeding. 
No joints exist between the thermojunctions and the cups of the mercury key, the 
platinoid wires of the thermojunctions being taken direct to the mercury cups; and 
wherever there are joints in this circuit they occur in pairs, and the corresponding 
joints are insulated and placed side by side, and then wrapped in a thick protective 
covering of cotton wool. All connections exposed to draughts, or likely to give rise to 
thermo-electric currents, were covered up with cotton wool. 
Galvanometer. 
11. Owing to the passing of electric trams in the vicinity of the Laboratory, we 
were obliged to abandon the dead-beat Thomson reflecting galvanometer of the old 
pattern (having a small mirror backed with four tiny magnets, hung by means of a 
spider line, at the centre of the galvanometer coil), which was used on former 
occasions and found thoroughly satisfactory. A Kelvin marine type of moving-coil 
galvanometer was substituted for it, and was made extremely sensitive and dead¬ 
beat ; and was also adjusted to give proportional deflections. This, used with a 
Steinheil telescope, placed 1 metre distant from the galvanometer mirror, and with 
* To prevent the thermojunction wires from touching one another in the liquid air bath, the platinum 
wire is enclosed in a piece of thermometer tubing, and the platinoid wire is tied at intervals to the outside 
of the tube. This allows the junction to be moved about in the bath without fear of disturbing 
the wires. 
2 z 
YOL. CCVIII.-A. 
