92 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
conveniently built up around the liea.vy gunmetal frame carrying the controlling 
system. The iron slabs of the magnetic screen were also tin-foil coated and a roof 
was erected on them of mill-board covered with tin-foil. This roof had holes cut in it 
for the connecting wires to come through, and for the stem of the controlling system. 
All these precautions were the result of attempts to remedy faults as they made their 
appearance. 
Magnetic Screen. —Four iron slabs were prepared from cast iron and arranged so 
as to form a symmetrical box, without top or bottom, round the instrument. The 
screening was not very perfect, and if we could have got soft iron we would have 
used it. However, the improvement made by this screening was very great, and it 
was no use trying to improve it by cast iron, for we used as much as we could trust 
the concrete slab to carry, and put it as near to the instrument as possible. It was 
supported on a wooden frame. When further improvements are made, the most 
important of them will be in increasing the magnetic screening, even if it takes half a 
ton of soft iron. The approximate reduction of the earth’s horizontal field due to our 
.screen is from '2G C.G.S. to about one-twentieth of this. The screen weighed 
300 lbs. 
Residual Fluctuations of Zero. —These always make their appearance when the 
single steady deflexion is above 1 micrometer division for 5 X 10" 11 ampere. We 
have endeavoured to trace the cause of this fluctuation. We made a number of 
experiments by heating the instrument unequally by means of gas furnaces placed in 
different positions with respect to it, with the result that we discovered that the 
temperature differences had a considerable effect, but not enough to account for the 
fluctuations. We habitually use the instrument in a room of fairly constant 
temperature, and the method of illumination cannot be charged with giving rise to 
the variations; for the lamp may be turned up so as to radiate, say, a hundredfold 
as much energy without producing any increased effect. The only radiation reaching 
the interior from the lamp is that which can pass through the fine scale divisions, and 
this all impinges on the surface of the mirror. The fact that the instrument is 
practically air-tight, precludes the theory that external currents of air cause the 
effect. Besides, there is the mill-board box coated with tin-foil, and outside that the 
iron and mill-board box. 
It remains to consider variations in external magnetic force, and it is to these 
variations that we attribute the fluctuations observed. Our reasons are: (l) The 
magnetic screen made the needle much steadier. (2) The residual unsteadiness varies 
very much from day to day ; on some days it is impossible to use the instrument with 
a period of more than 10 sec., and on others the zero is absolutely still and constant 
during, say half-an-hour, when the period reaches 25 sec. or 30 sec., and, ol course, it 
always happens that these periods of magnetic peace only occur when we want to 
measure comparatively low resistances. The fluctuations have, moreover, all the 
appearance of being due to general causes, for they appear equally at night-time when 
