4? 5 2 Dr. Draper on the Electro-motive Former of Heat, 
cups of the galvanometer. The coil of this galvanometer is 
of copper wire Jth of an inch thick^ and making twelve turns 
only, round the needles, which are astatic. The deviations 
were determined by the torsion of a glass thread, in the way 
described in the number of this Journal for October 1839. 
It is surprising to those who have never before seen the 
experiment, with what promptitude and accuracy a copper 
and iron wire, soldered thus together, will indicate tempera- 
tures. 
In the arrangement now described, when an experiment 
has to be made, the vessel A A is to be filled two-thirds full 
of water, the bulb of the thermometer being so adjusted as to 
be in the middle of the vessel, and the soldered extremity s 
of the two wires, being placed in contact^ with it, and a 
small cover with suitable apertures adjusted on the top of the 
vessel, so that the steam as it is generated may rush up along- 
side of the tube of the thermometer, and bring the mercurial 
column in it to an uniform temperature. The communicating 
wires / f are then placed in the cups, and the trough e filled 
with water and pounded ice, and carefully surrounded with a 
flannel cloth. The water in the vessel A A, is then gradually 
raised to the boiling point by means of a spirit-lamp, and 
kept at that temperature until the galvanometer needles and 
the thermometer are quite steady. The same plan must be 
followed, when any other temperature than 212 is under trial, 
for the thermo-electric wires changing their temperature more 
rapidly than the mercury in the thermometer, it is absolutely 
necessary to continue the experiment for some minutes, to 
bring both to the same state of equilibrium. 
When a temperature higher than 212° Fahr., but under a 
red heat, is required, I substitute in place of the vessel A A, 
a tubulated retort, the tubulure of which is large enough to 
allow the passage of the bulb of the thermometer and the 
wires. A quantity of mercury, sufficient to fill the retort half 
full, is then introduced, and the tubulure, being closed by ap- 
propriate pieces of soapstone, the neck of the retort is in- 
clined upwards, so that the vapour as it rises may condense 
and drop back again, without incommoding the operator. 
As in the former case, it is here also necessary to continue 
each experiment for a few minutes, to bring the thermometer 
and thermal pair to the same condition. There is not much 
difficulty in obtaining any required temperature, by raising 
or lowering the wick of the lamp. 
* If the extremity of the thermo-electric pair be allowed to rest on 
the bottom of the glass vessel, no accurate results can be obtained; the 
|:air does not then indicate the temperature of the water. 
