RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
83 
Table XIX.—Giving the Intensities of the Axial Magnetic Fields produced by a 
Current of 2 Amperes in Two Solenoids One of which has the Motor as a Core. 
Axial distance from 
centre of coil. 
Intensity of magnetic field. 
No iron present in coil. 
Motor as core of the coil. 
cm. 
C.G.S. units. 
C.G.S. units. 
100 
0-023 
0-09 
200 
0-0034 
0-013 
300 
0-0009 
0-0036 
400 
0-0003 
o-ooio 
The motor was not running during these measurements. 
been stated to cause no trouble. The intensity of the component of this stray field 
parallel to the shaft of the apparatus and 400 cm. from the centre of the motor, was 
measured to be 0'0006 C.G.S. units. 
The general conclusion is that an electric motor may safely be used for such a 
purpose as ours. 
Section 20.—Arrangement of Circuits. 
(a) Connection to Brushes. —The brushes are connected to insulated terminals fixed 
to the phosphor-bronze rings, and double silk-covered copper connecting wires pass 
from these terminals to a selector switch on the observation table. A diagrammatic 
representation of the connections is given in fig. 19. The selector switch consists of 
two conducting arms which enable the observer to complete the circuit through any 
one of the five pairs of brushes by moving the contacts from stud to stud. When it is 
desired to place the ten brushes in series and thereby get an induced voltage five 
times as great as with one pair of brushes, conducting straps are placed in the 
positions SS SS indicated by the dotted lines and the turning head makes contact 
with the studs V and 5. Any one of the five pairs of brushes may still be selected 
and observations may be made to test if the brush contacts are satisfactory. When 
the ten brushes are placed in series the thermo-electric effects are of course added 
and particular care has therefore to be taken with the brushes. When trouble is 
experienced it greatly facilitates the work to select pairs of brushes and so detect the 
faulty ones. 
To place the brushes in two sets of five in parallel, conducting straps connect 
together each set of five terminals TT, &c., on the selector switch. The position of 
the turning head is immaterial. 
(b) Multiple Commutator and Plug Board (fig. 19).—This is designed after the 
manner of the multiple commutator used for the Ayrton-Jones current weigher, and a 
short description will therefore suffice. A commutator allows of the reversal of the 
M 2 
