46 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
piece of circular lamp wick about 3 cm. long is slipped ; the lower end of this wick 
just touches the rotating segments and thus feeds petrol directly on to the surface of 
the segments. The usual rate of supply of petrol was about 500 c.c. in twenty 
minutes. When the apparatus is running well, an observer may place the end of a 
finger on the rotating segments and find it well flooded with petrol removed from the 
rim. The wicks are renewed at least every day and the brushes wiped with clean 
chamois leather. In our experiments a set of ten brushes lasted usually for six or 
nine complete sets of observations; after that number the surfaces of the wires 
became somewhat rough and the results were not quite so satisfactory. The 
magnitude of the changes of the thermo-electric effects with two sets of five brushes 
in parallel will be realized when it is said that often for intervals of twenty minutes 
the rapid variations in the total thermo-electric voltage did not exceed 0'1 microvolt. 
A slow progressive variation was commonly observed, but this was not a source of 
trouble. 
Section 8.—The Coil Supports. 
Each marble cylinder weighs about 50 kgr. and is supported on a phosphor-bronze 
cradle C (fig. 4) so that its axis is coincident with the axis of the shaft. The base of 
the support is a triangular casting G, which is supported in turn by three levelling 
screws A on the “ hole, slot, and plane ” principle. The pitch of the levelling screws 
is one millimetre and the heads of the screws are divided into one hundred equal 
divisions, thus enabling any particular vertical motion to be repeated within one- 
hundredth of a millimetre. 
A second triangular casting K carries the levelling screws and is fitted over a large 
central stud attached to a slide, so that the cylinder may be rotated about a vertical 
axis if necessary. The maximum angular motion is 12 degrees and the magnitude of 
any motion can be directly read on the engraved head of the horizontal screw E to 
half a minute of arc. Backlash is avoided by the use of strong phosphor-bronze 
springs. 
A cylinder and the cradle supporting it can be moved 5 cm. in two horizontal 
directions at right angles by means of two slides, the motions being controlled by screws, 
the heads H of which are divided to read hundredths of a millimetre as in the case of 
the screws for the vertical motion. For these movements also strong phosphor-bronze 
springs are employed to prevent backlash. 
Section 9. —Lubrication of Bearings. 
The bearings of the machine are nine in number, each of those of the motor being 
about 9 cm. long and each of the remaining seven about 14'5 cm. long. The lubricant 
used is best turbine oil which is fed to the bearings under a pressure of about 
15 lbs. per square inch, the rate of supply being a cubic foot of oil every five minutes. 
The oil supply tank and pump are of phosphor bronze and are situated under the 
