44 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
copper wire, and solid slipper brushes of phosphor bronze were also used. The most 
satisfactory results were obtained with brushes of fine wire. 
After the erection of the Lorenz apparatus we continued our experiments with 
brushes. The discs of the new apparatus are about 53 cm. in diameter and they 
revolve about 1050 times per minute, so that the velocity at the edge of a disc is 
about 1750 m. or 1 mile per minute. This velocity is greater than the velocity at 
the edges of the discs in our preliminary apparatus, and we found the brush difficulties 
correspondingly increased. The variation in the resistance of the galvanometer 
circuit due to the varying contact resistances rendered accurate work impossible, and 
we were led to design a brush which would not be set in vibration except to a very 
small extent and by the use of which thermo-electric effects would produce compara¬ 
tively small disturbances. A short description of these brushes appeared in the 
‘Annual Report of the National Physical Laboratory for 1911,’ and we are aware 
that the short description there published has led several investigators to try similar 
brushes, and the results have been reported as satisfactory. 
We were guided in the design by the following considerations : (l) a fine cylindrical 
wire if stretched offers little resistance to a stream of air, and whatever vibration is 
set up will probably be regular and can be controlled by adjustment of the tension on 
the wire ; (2) if a wire such as AB (fig. 6) is in tension and makes- contact with the 
edge of a rotating disc D over the arc ECF, the pressure will be greatest at a point 
close to C the mid-point, and will gradually fall in value until at E and F it is zero. 
The maximum rise of temperature and the maximum thermo-electric effect will 
probably be at C, and the thermo-electric force will gradually diminish as we approach 
E and F and be zero at these points. Without the completion of any other circuit it 
is clear that electric currents will be produced which will flow from the wire to the 
disc in some parts and from the disc to the wire in other parts. If, therefore, a 
galvanometer is included in the circuit containing the junction of wire and disc, the 
resultant deflection will be very much less than that which would be produced by the 
maximum thermo-electric effect. A number of stretched wires in parallel should, of 
course, give better results than one. The form of brush actually used in our 
experiments is depicted in fig. 6. The size was largely governed by the arc of contact 
desired and the number of segments on the discs. 
