12 Mr. C. Barus on the Fusion 



the room temperature both decreases the effective magnetic 

 field and increases the resistance of the coil, by a combined 

 amount probably greater than 0*4 per cent, per degree. 

 Hence in a room of variable temperature (the usual case), the 

 torsion-galvanometer of unknown temperature- coefficient is 

 liable to lead to serious errors. 



Results. 



12. Method of Computation. — I will pass over this here, for 

 the forms to be given to the cumbersome equations depend 

 upon the special purpose of the observer. Suffice it to say 

 that I divided the total gas volume into 4 parts, viz., the 

 volume of the bulb and hot stem, the volume of the part of 

 the stem where temperature falls off from hot to cold, the 

 volume of the cold stem and capillary, and finally the volume 

 measured in the manometer. The effect of each of these 

 parts on the final result w r as carefully evaluated. 



13. Bulb Volumetry. — The volume of the bulb at zero 

 Centigrade, which enters fundamentally into the constant- 

 pressure method, can be obtained by water calibration, cf. 

 § 18. In the case of a manometer like the one described 

 above, § 7, in which pressure can be varied over a sufficiently 

 wide range and volumes read off with facility, the bulb- 

 volume may also be obtained by air volumetry. For instance, 

 if v and p be corresponding values for the volume of air in 

 the manometer and the pressure, the following values were 

 obtained for the bulb volume, i' : — 



p =75-24, 52-64 ; 75-22, 53-41 ; 75-54, 53-38 ; 53-38, 75-53 

 v = 4-0, 126-0 ; 4-0, 120-5 ; 3-0, 120-5 ; 120-5, 3-0 



281-3 c.c. 279-9 c.c. 281-3 c. c. 



After much further experimentation I convinced myself 

 that the differences here are simply thermal discrepancies, 

 and are not due to the non-glazed interior surface of the 

 bulb. If the high-temperature datum is to be measured with 

 an absolute accuracy of 1° in 1000°, the v must be known 

 with a degree of precision scarcely exceeding *02 per cent., 

 i. e. to about 0*1 c. c. in the above case. Supposing sufficiently 

 sharp means (accurate to o, 05 C.) for measuring or con- 

 trolling temperature to be at hand (the above data were made 

 with the manometer in air), the stated accuracy is attainable. 



14. Coefficient of Expansion of Porcelain. — The porcelain 

 of Bayeux, in addition to its refractory qualities, has an 



