the Magnetism of Basalt. 49 
the current supplied to the magnetizing-coil, and C; that due 
to the furnace current. The current in the magnetizing-coil 
was measured by the amperemeter A, and regulated by the 
rheostat R. The resistance measurements were made by 
means of the box K MN, along with the divided ohm O O, 
the bridge-wire W W, and the galvanometer G, with its 
scale §; Bisa Leclanché cell and D a resistance of 60 ohms. 
An observer standing near the key H could thus take all the 
readings, the eotnmutane E being manipulated by two threads 
passing between E and H. 
For the platinum thermometer a length of about 110 cms. 
of pure platinum wire, 0°15 mm. in diameter, obtained from 
Messrs. Johnson & Matthey, was wound on a flat strip of 
mica and connected with thicker leads, compensating leads 
being also employed. The strip of mica on which the 
thermometer was wound was nearly the same length as the 
bars of basalt, so that the temperature measured would be 
the mean temperature of the part of the furnace occupied by 
the bar. No trouble was experienced from laying the bare 
thermometer on the basalt, and measurements of the surface 
conductivity of the bars before and after heating showed 
that, when cold at least, the insulation was quite sufficient. 
Further, the constants of the thermometer did not change by 
any greater amount than could be attributed to the handling the 
latter underwent in being placed in the furnace at the beginning 
of the experiments. Resistances were measured by means 
of an old but very accurate Elliott B. A. box combined with 
a divided ohm and a metre bridge, the values of the two 
latter being expressed in terms of the Elliott box 1 ohm coil. 
The Elliott box was one which had been used and standard- 
ized by Mr. R. H. Housman, and its 1 ohm coil, when 
compared with two standard B. A. units, was found to be 
~ correct to the fourth decimal place. The thermometer was 
standardized and temperatures reckoned according to the 
methods recommended by Callendar and Griffiths *, except 
that boiling mercury was employed to determine the third 
fixed point instead of boiling sulphur. The values obtained 
for the constants gave a value of 6 closely approximating to 
1°57, and the latter constant was therefore employed. 
The thermometer was first used to find to what extent the 
temperature within the furnace might be taken as uniform, 
and the latter was explored while being heated by 25 and 48 
accumulator cells respectively. The results. showed that at 
* H. L. Callendar, Phil. Trans. p. 161 (1887); Callendar & Griffiths, 
Proc. Roy. Soc. vol. xix, p. 56 (1880) E. H. Griffiths, ‘ Nature,’ vol. lil, 
p. 39, Nov. 14th, 1895. 
Phil. Mag. 8. 6. Vol. 7. No. 37. Jan. 1904. 1D) 
