452 Dr. Draper on the 'Electro-motive Power of Heat. 



cups of the galvanometer. The coil of this galvanometer is 

 of copper wire §^th of an inch thick, and making twelve turns 

 only, round the needles, which are astatic. The deviations 

 were determined by the toi'sion 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 

 yi'wQsff 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 

 pair does not then indicate the temperature of the water. 



