IRON AND STEEL IN A ROTATING MAGNETIC EIELD. 
723 
’WnH 2 
alternating magnetic field = 1'67 ^ - I6 , the difference in the constant being due to 
the fact that the distribution of E.M.F. is different in the two cases, the average 
being larger for the rotating field, and also that in the latter the average length of 
path and resistance of the eddy current circuits are smaller. 
It is more convenient to estimate the loss due to eddy currents in ergs per 
3B 2 w£ 2 
revolution. This = fly - er £ s P er cu ^- cen fi m - per revolution. 
For the soft iron t — ’0081 and p = 1 ’2 X 10 -5 ; .*. eddy losses = 1*64 X 10 -8 Wn. 
For the hard steel t = '0141 and p = 1‘5 X 10~ 5 ; .*. eddy losses — 4’0 X 10“ 8 B hi. 
Determination of Induction in Armature. 
The induction in the armature was measured by winding a few turns of wire 
round the iron, through holes in the ebonite washers. The electro-magnet was 
excited by a measured current. The coil was placed in series with a ballistic 
galvanometer, and the throw observed when the coil was suddenly moved half 
round. By this means all effects of the stray field of the magnet on the galvano¬ 
meter and leading-in wires were avoided. These were, however, very small, since 
the galvanometer was 30 feet distant, and the connecting wires were carefully stranded 
and insulated. 
The galvanometer was standardised by a Clark s standard cell and a Muirhead 
standard condenser. The resistance of the galvanometer circuit and coil was made 
large, to avoid damping due to currents induced in the coils by the swing of the 
needle. Other causes of damping being the same both for the induction throw and 
the condenser throw, it was not necessary to allow for this. In the soft iron 
armature, and to a less extent in the steel armature, the paper occupies some space, 
and with a strong field the lines of force through the paper are an appreciable 
quantity. The strength of the field was determined by winding an air coil round 
the ebonite washers at the end, and measuring the throw. This gives a fairly 
accurate value for the strength of the field, as it is just outside the iron. The area 
occupied by paper and air in the armature was calculated, and the proportional 
correction subtracted from the total number of lines of force as given by the coil round 
the iron. It did not amount to more than 5 per cent, in the strongest field. The 
results of the calibrations are given in the Diagrams 2 and 3, from which the values 
for the induction in the tables of readings are taken. 
In the value of the area of the iron, the small hole in the centre was subtracted 
from the total cross-section of the armature, so that over a part of the armature the 
value of B will be really somewhat less than that given. But for the part under the 
pole-pieces the value will be nearly correct. 
4 z 2 
