92 



Mr. Henry Crew on a New Method 



as with the current strength, or the resistance of the wrapping, 

 the steady flow of heat obeying Ohm's law. 



For the recognition of a constant current one cannot use 

 an ordinary tangent-galvanometer, since, while variations in 

 the direction of H can be eliminated by commuting the cur- 

 rent through the galvanometer, the variations in the intensity 

 of H cannot be so eliminated. Neither is a D'Arsonval gal- 

 vanometer available ; for there is a time-variation in the 

 rigidity of the wire suspension, and a temperature-variation 

 in the field of the permanent magnet. Not only so, but such 

 an instrument must be used with a shunt, and unless the 

 shunt has the same temperature-coefficient as the suspension- 

 wire and coil of the galvanometer, the shunt ratio will vary 

 with the temperature. However, if one has no better instru- 

 ment at his disposal fair results may be obtained with this. 

 A Thomson centiampere or milliampere balance, being in- 

 dependent of thermal and magnetic changes, and needing no 

 shunt for ordinary purposes, is an ideal instrument for this 

 work. 



The rheostat, r, should be placed immediately in front of it 

 for convenience. 



An additional convenience is obtained by placing a re- 

 latively small resistance in series with the Thomson balance, 

 so arranged that it can be shunted out at will. In this way 

 the current can be temporarily increased. Otherwise it will 

 take a long time for the condition of steady flow T to be 

 reached. 



A temporary increase of this kind is what is meant, in the 

 observations which follow, by the note, " current temporarily 

 increased." 



Observation I. 



Time of 

 Obs. 



Reading 



Current on 



of 



D'Arsonval 



Thermometer. 



Galvanometer. 



h m 



o 



div. 



3 10 



38-10 



8-350 



3 20 



3840 



} , 



Current 



interrupted. 





3 47 



37-87 



8-350 



3 57 



37-86 





4 05 



37-87 



,, 



4 15 



37-87 



>> 



4 25 



37-87 





4 31 



37-87 



5) 



4 49 



37-875 



» 



