RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
51 
The samples of Portland cement tested varied in magnetic permeability from 1'0005 
to 1'0020. It is possible that metallic iron finds its way into the cement from the 
steel grinding machines, and the Associated Portland Cement Manufacturers very 
kindly offered to exclude such contamination by grinding some cement through mill¬ 
stones. At the same time the manufacturers pointed out that a certain amount of oxide 
of iron in combination with lime and silica is invariably present in Portland cement, 
and Keene’s cement was suggested as being suitable for our purpose. This is a 
white cement absolutely free from iron compounds of any sort. We tested several 
samples for magnetic quality, and finding it quite satisfactory decided to use it for 
the block on which the apparatus rests. No trouble was experienced in obtaining 
sand and ballast free from magnetic substances. 
Section 13.— Construction and Measurement of the Coils. 
“ First Statuary ” Carrara marble was chosen for the material of the cylinders. We 
were guided in our choice by our experience with the cylinders for the Ayrton-Jones 
current balance, the tests then made showing marble to be an excellent electrical 
insulator and of negligible magnetic susceptibility. 
The cylinders were prepared in the rough by Messrs Walton, Gooddy & Cripps, whom 
we thank for the trouble they took in choosing masses of marble practically free from 
veins. The small shiny specks which are often present in Carrara marble consist of 
iron pyrites which has a magnetic susceptibility of about 0‘0005. The conductivity 
of iron pyrites is much greater than that of marble and any small specks on the 
surface of a cylinder should therefore be removed; we found that a small crystal of 
pyrites pressed between two plates of copper reduced the insulation between the 
plates from a value which was practically infinity to 1000 ohms. Clearly a large 
number of crystals at the surface of a marble cylinder would introduce a serious source 
of error. 
The coefficient of expansion of the marble was determined by direct measurements 
on the cylinders and found to be 5‘0 x 10~ 6 for 1° C., the temperature range being 
from 11° C. to 20° C. These measurements are described in the section dealing with 
the measurements of the mean diameters of the coils. 
The cylinders chosen are free from flaws and cavities. Of six cylinders which were 
submitted, two exhibited “ ground flaws ” and were in consequence rejected. These 
“ ground flaws ” appear to have been produced by a r.upture in the marble many 
thousands of years ago and subsequent re-union by pressure or equivalent agency. 
In all, Messrs Walton, Gooddy & Cripps prepared 16 cylinders in the rough and the 
four best of them were chosen for our work. 
The marble cylinders for the Ayrton-Jones current balance were baked in an oven 
at 140° C. for 30 hours and afterwards immersed in hot paraffin wax. At the same 
time a marble rod was subjected to similar treatment, and since that time this rod has 
H 2 
