66 
ME. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
(p. 85, fig. 19) was removed, and the voltage applied to the circuit was the same 
as that used for the resistance measurements. The insulation resistance was always 
greater than 1000 megohms. 
To measure the insulation resistance between the coils, the latter were usually arranged 
in two groups, each group consisting of one coil from each cylinder. At first, with an 
applied voltage of 20 volts, the insulation resistance was about 10,000 ohms. The 
insulation resistance of coils on cylinders Nos. 2, 3, and 4 proved to be well above 
1000 megohms, but that on the coils on cylinder 1 was low. We found these coils to 
be in close proximity to, or in contact with, a few crystals of pyrites, and after 
dislodging these, wholly or in part, the insulation resistance increased to above 1000 
megohms. No trouble has since occurred. As the coils are of bare copper wire and 
are covered only with a thin wrapper of silk, we think it necessary to make an 
insulation test of the coils on each day that resistance measurements are made. The 
test occupies but a few minutes, and during our work it was regularly made. 
The insulation resistance between the earth and the rotating wires attached to the 
discs was usually tested at 20 volts. The results in all cases were satisfactory. For 
certain measurements, it is essential that the rotating wires be insulated from each 
other and as the wires may not occupy exactly the same positions with respect to the 
disc and shaft when rotating, as when stationary, the tests are preferably made with 
the machine running. The test when the wires are rotating is of some interest. One 
terminal of the galvanometer is connected to one brush, and one pole of the battery to 
the remaining four brushes, the five brushes being in contact with the segments on 
one of the discs. The other terminal of the galvanometer is connected to the remaining 
pole of the battery. From fig. 5, given on p. 42, it is clear that when the discs are 
rotating one terminal of the galvanometer is always connected to one or two of the 
rotating wires, and the other terminal, through the battery, is connected sometimes 
with four and sometimes with eight of the wires. The wires are continually changing 
in position and the deflection of the galvanometer enables the insulation resistance to 
be calculated. The first set of conducting wires which were used became faulty in 
insulation resistance because of the action of some insulating tape which was used to 
bind the wires together. They were replaced by others which were supplied with 
double layers of silk and which we subsequently shellacked and encased in silk tubes. 
No trouble has since been experienced, the insulation resistance being well over 200 
megohms. The insulation of the remaining part of the circuit, e.g., leads to brushes, 
standard resistance, &c., was frequently tested, hut no fault was found. 
Section 16.— Measurement oe the Diametral Distance between 
Opposite Segments. 
The distance between opposite segments was measured both when the discs were 
stationary and when running at speeds varying from 170 to 1110 revolutions per 
minute. 
