530 
MR. O. W. RICHARDSON ON THE ELECTRIC AT. CONDUCTIVITY 
taken to correspond to the melting-point of the salt. The method was then tested 
by placing a small portion of salt on the thermocouple itself and observing when it 
began to melt. This Avas found to agree with the previous observations. The 
greatest difference between the observations taken was less than 3 per cent. A 
further test Avas supplied by determining the melting-point of sodium sulphate ; the 
value found Avas within 20° of th.it giA^en by Messrs. Heycock and Neville.* This 
agreement Avas considered to be quite good enough for the purpose in hand. The 
thermocouple was finally found to give an electromotive force of 17‘7 milliAmlts Avhen 
its junctions AA^ere at 1067° C. and 12° 0. respectively. 
The platinum temperatures given by the galvanometer readings have been corrected 
to the air thermometer scale by means of the curves given by Professor CALLEXDAE.f 
The thermocouple method possesses one great advantage OA^er the resistance method 
of determining the temperature of hot Avires, in that the observations can be taken 
much more quickl}^ and so tlie Avire has to lie heated for a much shorter time. In 
this way the apparatus need never get hot, and far less gas is given off, so thot the 
readings generally are much steadier. 
The accompanying table represents a series of observations Avith this apparatus. 
The pressure was always less than of a millimetre of mercury, while the 
potential on the wire was — 44 volts, this being more than enough to saturate the 
current. The platinum temjieratures are given under the column headed Pt, the 
numbers under t are the temperatures (degrees Centigrade) reduced to the air 
tliermometer scale. 
Scale-division.? of 
thermocouple. 
Ft. 
f. 
Leak, 
1 = 10~® ampere. 
Pressure. 
1 
uiilliins. 1 
108-5 
1122 
1110 
3-7 
o 
o 
110-8 
1145 
1129 
8-2 
— 
112-8 
1165 
1145 
25 
— 
■ 114-8 
1186 
1162 
39 
-001 
117 
1209 
1180 
78 
-- 
119 
1229 
1197 
167 
-0015 
120-6 
1245 
1209 
295 
— 
121 -5 
1254 
1216 
662 
— 
119-2 
1231 
1199 
266 
•0015 
117-3 
1212 
1183 
110 
— 
116-3 
1202 
1173 
79 
— 
113 
1168 
1148 
37 
— 
110-7 
1144 
1128 
16-5 
— 
109 
1127 
1115 
7-5 
— 
107 
1107 
1097 
3-7 
— 
104-1 
1077 
1075 
1-5 
•0016 
It will be noticed here again that the current for a giA’en temjierature is smaller as 
the temperature is being increased than AAdien it is falling. 
* ‘ Chem. Soc. Journal,’ vol. 67, p. 160. 
t ‘ Phil. Mag.,’ A'ol. 48, p. 519. 
