RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
55 
the N.P.L. values is of the same order. By taking the means of Sevres and N.P.L. 
determinations as the best obtainable, we have 
300+ 60 = 360 mm.-2'4/i at 0° C., 
200 + 160 = 360 mm.-O'b at 0° C. 
Using the dilatation equation determined at Sevres on a bar of the same material 
L t = L 0 {l + (l0'469 + 0'0035 2 £) t x 10“ 6 } 
we get the following lengths at 17° C. :— 
Gauges (300+ 60) = 360‘062 0 mm., 
„ (200+160) = 360‘064 0 mm., 
with an increase of 3‘8yu on the 360 mm. length per 1° C. increase in temperature at 
or about 17° C. 
To obtain the mean temperature of the marble, holes were drilled in the cylinders 
and in these holes thermometers were inserted. The holes are parallel to the axis of 
the coils, and the depth of each is about half the axial length of a cylinder, so that 
the temperature recorded by a thermometer is very approximately the mean 
temperature of the cylinder. 
The temperature coefficient of expansion of the marble was determined by making 
diametral measurements at temperatures varying from 11° C. to 20" C. Assuming 
that the coefficient of expansion of the steel 
gauges is correct, the mean coefficient of linear 
expansion of the marble was found to be 
5‘1 x 10 -6 for the range 11° C. to 20° C. The 
observations were remarkably consistent, as will 
be seen from fig. 9, in which the difference of 
expansion of the steel and marble is plotted 
against temperature. During these observa¬ 
tions we were much struck with the compara¬ 
tive rapidity with which the temperature of the 
marble followed that of its surroundings. The 
temperature coefficient of expansion of the 
marble was also deduced from measurements 
of the axial length of a cylinder. In this case 
10 
12 . 
14 16 18 
TEM PERATURE 
22 
Fig. 9. Differential expansion, steel-marble. 
the difference of expansion of invar and marble was directly recorded, and taking the 
invar as known the value obtained for marble is 4'9 x 10 ~ 6 , which is in good agreement 
with the value given above. When required in our work we took the mean of 
these values, i.e., 5'0x 10 -6 , as correct. 
As already stated, the contact pieces of the measuring machine are of agate, in 
