DR. J. HOPKINSON ON THE MAGNETISATION OF IRON. 
467 
Let AKB be the curve connecting A and B when the magnetising force is reversed, 
NLA when it is again reversed in this cycle ; the final magnetisation is the same as it 
was initially; hence the balance of work done upon the field must be converted into 
heat; this heat will be represented by the area AKBLA-p47t in ergs, per cubic 
centimeter. 
An approximation to the values of this dissipation is given in the table of results. 
It may be worth while to call attention to their practical application. Take the case 
of a dynamo-machine with an iron core, finely divided to avoid local electric currents. 
Note that we are going to assume—though whether true or false we do not know—- 
that the dissipation is the same whether the magnetisation is reversed by diminishing 
and increasing the intensity of magnetisation without altering its direction, or whether 
it is reversed by turning round its direction without reducing its amount to zero. 
A particular machine has in its core about 9000 cubic centims. of soft iron plates ; 
the resistance of its armature is 0‘01 ohm, of its shunt magnets 8’0 ohms, and when 
running 900 revolutions per minute, its E.M.F. at the brushes is 55 volts. When 
the current in the armature is 250 amperes we have 
Total energy of current. 
Loss in armature resistance .... 
Loss in magnet resistance. 
Loss in magnetising and demagnetising 
iron core of armature 
Ergs, per second. 
. = 144 X 10 9 . 
. = 625 X 10 7 . 
. = 378 X 10 7 . 
9000 cubic centims. X 
= *% per second X 13,356 
results) = 18 X 10 8 . 
15 revolutions 
(from table of 
From this we see at once that the heat generated in the core of the armature by 
reversal of magnetisation is about one-half of that arising from the resistance of the 
copper wire of the electro-magnet. If a hard steel were used the loss from reversal 
might amount to 20 per cent, or more of the useful work done. 
