OF THE NATIONAL PHYSICAL LABORATORY. 



was taken. The sulphur-points were always taken 

 on days when the pressure was not far removed from 

 760 millims. to eliminate the uncertainty as to the co- 

 efficient to use when reducing to normal pressure. 



XVI. Electric Furnaces. 



Two different electric furnaces were employed in 

 this work. Their dimensions were similar, hut they 

 differed in that in the first the heating-wire was 

 wound uniformly, and in the second an approximation 

 to a logarithmic spiral was made at each end, the 

 turns being gradually crowded, so that the cooling- 

 effect of the ends was in a great measure compensated 

 by the additional heat supply. Both furnaces were 

 wound with wire of pure nickel about 1 '6 millims. 

 diameter. The heating current was supplied from a 

 special battery of 56 accumulators reserved for this 

 purpose, which was divided into four groups of 14 

 cells, capable of being coupled in series or parallel, 

 as desired. A set of large well-ventilated manganin 

 resistances, formed of two No. 9 wires in parallel, and 

 capable of carrying 100 amperes without undue heat- 

 ing, was arranged so that the external resistance of 

 the circuit could be altered by steps of '025 ohm up 

 to 3 '2 ohms, thus enabling any desired amount of 

 energy to be put into the furnace at will. The con- 

 struction of the furnace and the disposition of the 

 different instruments within it is shown in fig. 7. 

 The nickel heating-wire is wound upon the inner tube 

 of unglazed biscuit porcelain, and in order to prevent 

 the turns becoming short-circuited when hot, the 

 whole of the wound portion is covered with a thin 

 layer of " purimachos " which is baked on at a 

 moderate heat. The leading-iii wires are doubled 

 or trebled in all cases. The bulb of the gas thermo- 

 meter is supported on a small bridge of fire-clay 

 resting on the furnace bottom, and the standard 

 platinum thermometer and thermoj unction are 

 arranged as shown, great care being taken that 

 neither the wires of the junction nor the thin porcelain 



VOL. com. A. 3 A 



361 



