PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 707 
These results seem on the whole to show that the mean of apparent temperatures 
giving no current is rather less for the wide than for the narrow ranges, in the case of 
the two metals concerned ; that is, that the mean of the apparent temperatures giving 
no current is somewhat below the true neutral point. I need scarcely remark, however, 
that even if this indication could be relied on, it would be necessary to compare the 
actual mercurial thermometers which were used, with an air-thermometer, before any 
conclusions of value could be drawn from it regarding the constancy of the difference 
of specific heats of electricity in lead and platinum. 
100. The following Table shows the results of observations leading to actual deter- 
minations of neutral points between various pairs of metals — 
-14° C. 
-12°*2. 
-5°*7. 
-3°-06. 
- 1°-5. 
8°*2. 
33°. 
36°. 
38°. 
44°. 
1 
0 
0 * 
0 
P3 
Brass. 
Pi 
Cadmium. 
Silver. 
Gold. 
Pi 
Gold. 
Pi 
Silver. 
Pi 
Zinc. 
Tin. 
Brass. 
P2 
Lead. 
P2 
Brass. 
P2 
Tin. 
Lead. 
Brass. 
Different specimens of Silver. 
Different specimens of Zinc. 
53°. 
57°*. 
64°. 
71°. 
99°. 
121 °. 
130° 
162°'5. 
Some tempera- 
ture between 
223° & 253°-5, 
237°. 
280°. 
Double wire of 1 
Palladium 11*31 grs., ^ 
and Copper 19*41 grs.J 
Hard steel. 
Cadmium. 
Copper. 
Gold. 
Zinc. 
Brass, 
Pi 
Lead. 
^ 1 
Tin. 
Iron. 
Cadmium. 
Iron. 
Gold. 
Iron. 
Silver. 
Iron. 
Copper. 
The number at the head of each column expresses the temperature Centigrade by 
mercurial thermometers, at which the two metals written below it are thermo-elec- 
trically neutral to one another ; and the lower metal in each column is that which 
passes the other from bismuth towards antimony as the temperature rises. 
It was also found that Aluminium must be neutral to either Pg or Brass, or Pg, at 
some temperature between — 14° C. and 38° C.; that Brass becomes neutral to Copper 
at some high temperature, probably between 800° and 1400°; Copper to Silver, a little 
below the melting-point of silver; Nickel to Palladium, at some high temperature, 
perhaps about a low red heat ; and Pg to impure mercury (that had been used for 
amalgamating zinc plates), at a temperature between —10° and 0°. Pg appears to 
become neutral to pure mercury at some temperature below —25° Cent. 
101. The following Diagram exhibits graphically the relative thermo-electric bear- 
ings of the different metals, and may in fact be regarded as a series of tables of the 
thermo-electric order of metals at different temperatures from —30° to 300° Cent. 
* This determination has been added in consequence of information given by Mr. Joule (December 1856), 
that hardened steel at ordinary temperatures differs thermo-electrically from copper by about one-tenth of the 
thermo-electric difference of iron from copper. 
5 A 
MDCCCLVI. 
