146 Mr. Carl Bar us on the Pressure- Variations of 



millimetres, at extreme white heat. The crucible must be 

 well glazed internally ; and it is probable that the work will 

 be facilitated by using tubular forms made in a single piece, 

 L e. without the joint ef. In such a case the crucible ebb fa 

 may be reduced in size, and placed wholly ivithin the furnace, 

 the tube a leading out of it. The charge will then be more 

 effectually heated. A charge of granulated metal is easily 

 introduced through a. When normal boiling-points are 

 wanted, smaller crucibles of fire-clay are preferable "*. 



5. The thermoelectric powers can best be measured by a 

 null-method, in which the thermoelectric constants are 

 expressed in terms of a given Latimer-Clark's cell. Recali- 

 bration is then rarely necessary, and calibrations made at 

 different times may be compared. But the adjustments and 

 computations, if they occur in great number, are incon- 

 venient. In this respect the torsion-galvanometer f offers 

 decided advantages. I constructed an apparatus of this kind 

 in which the thermoelectromotive forces are directly ex- 

 pressed in terms of the twist of a platinum fibre. 



The description of the galvanometer must be omitted here. 

 Its chief points are an astatic, aperiodic magnetic system, 

 the needles (two bundles of four each) of which are of glass- 

 hard steel, consecutively annealed at 100° in order to secure 

 stability both against changes of temper and of magnetiza- 

 tion J. The instrument is 135 cm. high, and is read off by 

 the observer standing. The magnetic moment of each needle 

 (10 cm. long) being intense, only a moderately thin platinum 

 fibre (*018 cm,) suffices ; and as the needles are brought back 

 to the fiducial zero of the instrument by torsion of the wire, 

 the directive effect of the terrestrial or local magnetic fields 

 is quite eliminated. The torsion-circle is sharply divided in 

 single degrees ; the fractions are read off by telescope and 

 scale, a mirror being attached to the needle. Both telescope 

 and scale are compendiously fixed to the galvanometer. 

 Thermoelectromotive forces are therefore expressed in terms 

 of a fixed standard, the torsional rigidity of the platinum 

 wire, and hence they are comparable even after the lapse of 

 time. Any temperature between 100° and intense white heat 

 may be measured without further adjustment. These desi- 

 derata, added to convenient manipulation and the durability 



* For details, particularly with regard to superheating, see Bull. U.S. 

 G. S. no. 54, pp. 89 to 94. _ 



t The priority of adapting Coulomb's torsion-balance for thermo- 

 electric measurement is due to Schinz (Dingler's Journal, clxxv. p. 87, 

 1865 ; ibid, clxxix. p. 436, 1866). Cf. Bull. U.S. G. S. no. 54, p. 48. 



t Cf Strouhal and Barus, Wied. Ann. xx. p. 683, 1880 : or Bull. U.S. 

 G. S. no. 14. chap. vi. 1885. 





