RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
67 
For the stationary measurements, two micrometer heads were fixed to, but 
insulated from, two short upright rods of brass secured to a stout bar of the same 
metal, the distance between the contact faces of the micrometer screws being 
approximately equal to the distance between opposite segments. This gauge was 
supported on two uprights secured to the slide rails of the Lorenz apparatus and 
adjusted in position for the measurement of a diameter. Between the segments and 
the micrometer screws, vertical wires of phosphor bronze, similar to those used for the 
brushes, were interposed, and the measured distance was taken as equal to the 
distance apart of the segments plus twice the diameter of a wire. Contact between 
a segment and a micrometer head was indicated by the buzzing of a telephone due to 
the passage of a small alternating current through a circuit including the telephone, 
the micrometer head, and the segments. Four measurements were made on each pair 
of segments lying on opposite sides of a diameter and the mean of the 20 measure¬ 
ments was taken as correct. Such a series of measurements was frequently taken 
and show the wear of the segments, due to their friction with the brushes, to he 
comparatively slight. As examples, we give the results of some measurements made 
in December, 1912, and in June, 1913. 
Table YIII.—Giving the Diametral Distance between Opposite Segments. 
Segments. 
Disc No. 1. 
Disc No. 2. 
December, 1912. 
June, 1913. 
December, 1912. 
June, 1913. 
cm. 
cm. 
cm. 
cm. 
1 and 6 
53-581 
53-562 
53-556 
53-546 
2 7 
•579 
•566 
•556 
•547 
3 „ 8 
•579 
•569 
•550 
•546 
4 „ 9 
•579 
•573 
•556 
•549 
5 „ 10 
•584 
•570 
•554 
•560 
Mean. 
53•580 
53-568 
53-554 
53-550 
Y 
Mean. 
53-574 cm. 
53 - 552 cm. 
• 
Mean radial distance 1 
= 26 • 787 cm. 
= 26 - 776 cm. 
of segments . . J 
The standard of length used was an invar rod with approximately flat ends, and 
measurements on this rod were made in the same way as those on the segments. 
When the discs were rotating, contact with the segments was made by advancing 
a brush by means of a micrometer screw until a telephone indicated the completion of 
a circuit as in the previous measurements. The micrometer head made contact with 
the brush holder through the medium of a small steel hemisphere attached to the 
k 2 
