White — Melting Point Methods at High Temperatures. 475 



working chamber, and is therefore wound on the inside of a 

 cylindrical furnace tube;* (2) of the cylindrical tube which 

 supports the coil, which must be refractory, a fairly good 

 insulator of electricity at high temperatures, not apt to crack 

 from heat, and of a material which can conveniently be 

 plastered on and baked, in order to repair the defects which 

 inevitably develop in use at the higher temperatures. Good 

 thermal conductivity in this cylinder is not disadvantageous ; 

 indeed, it probably tends to prevent cracking from unequal 

 distribution of the temperature. The dense mixture of mag- 

 nesia and various other ingredients sold by the Harbison- 

 Walker Company under the name of magnesite, which was in 

 use five years ago, is still the best material we know for the 

 purpose. (3) The outer layers of the furnace are merely 

 heat-insulating material — a layer of an inch or more of cal- 

 cined magnesia powder around the furnace tube and a very 

 porous fire-clay outside of this and at the ends. The great 

 porosity of the fire clay has several advantages ; the clay is 

 less likely to crack, it insulates heat better, and can easily 

 be pierced with holes wherever desired for the insertion 

 of thermoelements or other apparatus. These furnaces can 

 be brought to red heat in less than 40 minutes by a cur- 

 rent of 25 amperes, but will bear 40 amperes at the start. 

 The working current at high temperatures is about 20 

 amperes (2000 watts). At 1600° a heating rate of 5° 

 per minute has been safely employed, 7° at 1500°, and 12° 

 or more at temperatures 100° lower. How much more than 

 this the furnace can stand we do not know, and the attempt to 

 find out would obviously prove rather expensive. The furnaces 

 are heated by storage batteries, which furnish a practically 

 constant voltage. The heating rate can easily be regulated so 

 as to vary but 2 or 3 per cent from minute to minute, or a 

 stationary temperature can be kept constant within a degree 

 for hours with comparatively little attention. The real limit, 

 however, to the effectiveness of the temperature control is the 

 difficulty of securing uniformity of temperature throughout 

 the working chamber. 



The extreme softness of platinum at high temperatures and 

 the tendency of the solid insulating and supporting materials 

 to shrink and crack call for the greatest simplicity and strength 

 in the form of construction, and practically compel the ordinary 

 furnace to take the form of a vertical cylinder, heated only on 

 the sides. The ends are always much colder and therefore any 

 body within the central cavity inevitably suffers from an 

 unequal temperature distribution, which is of course also 



*For the method of winding, see Arthur L. Day and J. K. Clement, this 

 Journal, (4), xxvi, 411, 1908. 



