82 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
The intensity of the magnetic force, at a point corresponding to the centre of the 
motor, which is produced by a current of 2 amperes through the four coils of the 
Lorenz apparatus, is about 0'0025 C.G.S. units. The iron of the motor becomes 
magnetised and the result is an increase in the total magnetic flux through the discs. 
We had to find the ratio of this increase to the flux produced by the current in the 
four coils. This ratio we have found in two ways. In both of these we magnetised 
the iron of the motor by winding around it a large solenoid of 16 turns of insulated 
copper wire. When a current was passed through this coil the magnetising field 
inside the solenoid was approximately in the same direction as the field due to the 
four coils, i.e., practically parallel to the shaft. The iron was magnetised and the two 
discs of the Lorenz apparatus were caused to rotate in the resulting field. Since the 
mean field in which one disc rotated was not the same as that in which the other disc 
rotated, a difference of potential was produced between the edges of the two discs. 
In our experiments we compared the galvanometer deflection produced by this potential 
difference with that produced by a current in the four coils of the Lorenz apparatus 
when the mutual inductance of the coils and discs was changed by a known amount. 
The result found is that when the iron of the motor is placed in a magnetising field of 
2'5 C.G.S. units, the total effective magnetic flux through the two discs is 1 part in 
10,000 of that due to the current in the four Lorenz coils. With such a small 
magnetising field as 0'0025 C.G.S. units, it was impossible to measure any effect, but 
the calculated effect is ^ ^^l * ^ ^ part in 10,000,000. 
2 5 
In the second experiment we wound on a framework of wood a similar coil to that 
surrounding the motor. Both coils were of 20 cm. radius and had 16 complete turns. 
When a current of 5 amperes was passed through the coil, the intensity of the axial 
field was measured by means of a magnetometer at distances of 1, 2, 3, and 4 metres 
from the mean plane of the coil. Similar measurements were made along the axis of 
the coil which surrounded the motor, the current through this coil being 5 amperes as 
before. The results are of interest and are given in Table XIX. 
The nearer disc of the Lorenz apparatus is about 400 cm. from the centre of the 
motor, and we may take the magnetic force due to the motor as uniform over this 
disc. The area of one face of the disc is 2200 sq. cm. so that the total flux through 
it due to the iron of the motor was 2200 (O'OOIO —0'0003) = 1'5. Had the 
magnetising force on the iron of the motor been that due to 2 amperes through the 
four coils of the Lorenz apparatus, the total flux would be reduced to 
(1*54 x 0*0025)/2*5 = 0*0015. The total effective flux through the two discs will be 
somewhat less than this, but the reduction is not important. When making a 
resistance measurement, the total effective flux through the two discs is about 24,000, 
so that the magnitude of the motor effect is 0*0015/24000 = about 6 parts in 
100,000,000. 
The intensity of the stray field of the motor when running at full load has already 
