The Theory of Automatic Regulators. 281 
taking part of the work, and that the automatic regulator is inserted, the 
resistance of the field winding is at this instant re-inserted and the voltage 
drops rapidly along a curve which is practically the inverted curve of the 
no-load characteristic. When the voltage has dropped to a value 
corresponding to line c"cl", the rheostat is again short-circuited and a 
rapid rise in the voltage once more occurs, until line c'd' is again reached ; 
and so forth. The voltage of the machine is thus oscillating between the 
Fig. 6. — Principle of Tirrill regulator. 
values c'd' and c"d", and as these variations are very rapid and possess 
small absolute values, they are not noticed. 
It will be obvious that the maximum excitation, when the rheostat is 
short-circuited, must be considerably in excess of the excitation at full load 
and normal pressure, as otherwise over-travelling and rapidity of action 
near full load would be impossible. The pressure oa should certainly not 
be less than 15 per cent, above the normal pressure cd at full load and 
