16(3 PROF. TROPTTOX AND MR. NOBLE ON THE MECTIANICAL FORCES .4CTING 
be hung’ with its plane north and south, tlien about 12 o clock in the da^ there 
would he no couple tending to turn it, because the aether drift due to the earths 
motion in its orbit round the sun is at right angles to the plane of the condenser; 
on the other hand, at any other hour, say 2 o’clock, there would he a couple making 
itself felt by a tendency to rotate the plane of the condenser into the position 
at right angles to the drift. 
Tlie effect to be looked for was an extremely small one, being a second-order effect 
only. In the earlier forms of the apparatus the calculated deflection was found to 
he entirely obscured by a number of accidental disturbing causes, and, in fact, the 
course of tlie experiments consisted mainly in eliminating tiiese disturbances one 
by one. 
The first difficulty arose from the preponderance of the electrostatic attractions 
between the suspended condenser and the walls of the containing case. To minimise 
these effects the condenser was protected by a metallic coating which A^as kept at 
the same potential as the case, all being earthed. 
The suspending wire was made the insulated teiminal, so that any electrostatic 
pull upon this wire could only produce a force acting through the centre of support, 
and thus the rotational effect would be eliminated, leaving only a translatory pull, 
which would not affect the result. There were still residual electrostatic actions, 
Avhich were traced to the point where the suspension Avas fixed to the condenser. 
At this point there AA^as necessarily a break in the metallic protecting coating fixed 
to the condenser, oAvino- to the fact that the earthed terminal could not be hi ought 
very close to the insulated terminal AAuthout breaking doAA'ii the insulation, io 
eliminate tliis effect, the protecting coating Avas prolonged u}) round the support in 
the form of a CAdindei', about 3 centims. Avide, and (> centims. high. The pull being 
noAA" lietAA'een the cvlnnler and the AAure. and therefore an internal force, there could 
lie no I’otation pi'oduced. Finally, it AA'as found necessary, for similar reasons, to 
shield the AA'ire as it passed through the top of the containing A^essel (or case in AAhich 
the condenser hung) 1 ly a thin brass tube 1 centim. AAude, passing AA'ell into the a essel, 
and at the same potential as the Avire. These precautions practically eliminated all 
electrostatic disturbances. 
As it AA'as necessai’A' to keep the condenser charged lor seA'eral minutes at a time, 
condensers insulated Avith paraffin AA'ax AA'ere found (jiiite useless lor the high 
potentials required (3()(M) Aoilts), any small leak probably increasing itself by melting 
tlie AAmx. Special condensers AA'ere then made in the folioAving manner ; 
CarefulK' selected mica plates, '011 centim. thick,- v'ere indiAudually tested to 
about 5000 A'olts. A disc of mica, 7‘7 centims. diameter, Avas coA'ered with tinfoil 
of 4‘5 centims. diameter ; the uncoA'ered jiortion aaus coated Avith a thin layer of 
shellac-A'arnish. The disc Avas then diied on a hot })late. Tavo plates prepared in 
this way AA'ere ironed together Avith a hot iron, and the whole condenser A\as built up 
in this manner. We thus had almost perfect insulation, for A\'hen charged the 
