342 
DRS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
Reducing these losses to percentages we have 
G= 
1-6 
115 
115 
And from I. the leakage through the zinc plat 
and iron base. 
Hence the two gaps account for. 
The zinc plate and iron base account for 
And the area between the limbs. 
Making a total loss accounted for .... 
Out of an observed loss of. 
= 1*4 per cent. 
= 7-0 
= 10-3 
2-8 
10-3 
7-0 
20-1 
24-24 
>5 
The leakage through the shaft and from pole-piece to yoke, and one pole-piece to 
the other by exterior lines, will account for the remainder. 
Effect of the Current in the Armature. 
The currents in the fixed coils around the magnets are not the only magnetising 
forces applied in a dynamo machine ; the currents in the moving coils of the armature 
have also their effect on the resultant field. There are in general two independent 
variables in a dynamo machine, the current around the magnets and the current in 
the armature, and the relation of E.M.F. to currents is fully represented by a surface. 
In well-constructed machines the effect of the latter is reduced to a minimum, but it 
can be by no means neglected. When a section of the armature coils is commutated 
it must inevitably be momentarily short-circuited, and if at the time of commutation 
the field in which the section is moving is other than feeble, a considerable current 
will arise in that section, accompanied by waste of power and destructive sparking. 
It may be well at once to give an idea of the possible magnitude of such effects. In 
the machine already described the mean E.M.F. in a section of the armature at a 
certain speed may be taken as 6 volts, its resistance 0'000995 ohm. Setting aside, 
then, for the moment questions of self-induction, if a section were commutated at a 
time when it was in a field of one-tenth part of the mean intensity of the whole field 
there would arise in that section, whilst short-circuited by the collecting brush, a 
current of 600 amperes, four times the current when the section is doing its normal 
work. The ideal adjustment of the collecting brushes is such that during the time 
they short-circuit the sections of the armature the magnetic forces shall just suffice 
to stop the current in the section, and to reverse it to the same current in the 
opposite direction. 
Suppose the commutation occurs at an angle X in advance of the symmetrical 
position between the fields, and that the total current through the armature be C, 
