of some Rocks at High Temperatures. 49 



To lessen oxidation of the furnace winding it was thickly 

 coated with fireclay cement, and while the latter was still 

 liquid several successive layers of asbestos millboard were 

 bound round the whole as tightly as possible. Outside this 

 is a lagging of asbestos- wool enclosed in a cylinder of several 

 sheets of asbestos millboard. The sketch indicates approxi- 

 mately the total thickness of the lagging. The winding is 

 put on in two separate equal circuits, the turns of one 

 lying between those of the other. The circuits may be con- 

 nected either in series or parallel to facilitate the current 

 regulation. As the energy to be supplied to the furnace 

 varies widely according to the temperature required, this 

 arrangement is very convenient. The spaces at either 

 end of the rock cylinder are filled with asbestos disks with 

 small holes bored in them to carry the necessary wires, the 

 spaces D outside the end of the copper tube are filled with 

 asbestos- wool. The heating-wire E runs almost the entire 

 length of the furnace and is connected by bind-screws to 

 stout leads as shown. Two other platinum wires F and G 

 are attached to D at the ends of the cylinder and are used 

 for measuring the potential difference between these points. 

 The furnace stands on a thick pad of asbestos millboard H, 

 and a similar one is laid over the top, care being taken to 

 exclude the circulation of air as far as possible. The wires 

 leading to the couples are not shown in the figure ; a pair of 

 platinum wires leading to the multiple couple enter at the 

 bottom of the cylinder, and a pair of wires, one platinum and 

 the other platinum-iridium, forming the single couple enter 

 at the top. 



The current which flows through E is supplied by a large 

 storage-battery and regulated by a rheostat. It is measured 

 on a Kelvin portable combined ammeter and voltmeter, the 

 potential difference between F and G being read on the same 

 instrument. In this way the rate of generation of heat in 

 the known length of the heating-wire inside the rock cylinder 

 is found. 



The furnace current is alternating, being obtained from 

 the Dublin City Supply. For all the limestone determinations 

 and most of those for granite, the current was transformed 

 down to 10 volts and the two furnace circuits connected in 

 parallel. In the highest temperature determinations with 

 granite, the circuits were connected in series and current at 

 40 volts employed. 



There are several advantages in using low voltage alternating 

 current for the furnace, the chief being that by so doing 

 insulation troubles are greatly lessened, and the danger of 



Phil. Mag. S. 6. Vol. 24. No. 139. July 1912. E 



