Unless there are special load problems such as severely temperature dependent heater resistance or transformer fed heaters then fast cycling is the preferred and less costly method. It also minimises radio frequency interference and supply voltage waveform distortion. However if lights share a long supply cable with the heater load they may be seen to flicker slightly at the fast cycle rate as the load pulls down the line voltage. Normal load ammeters will pulsate and specially damped meters can be used which average the current reading. The smoothest possible control with a normal ac supply is phase-angle firing whereby switching occurs every half cycle as in the normal lamp dimmer switch.
Power is turned up by advancing the switch-on time within each half cycle.
See Fig 3. This is also expressed as firing angle, being the number of degrees of phase angle between switch-on time and the end of the current half cycle. Even the liveliest tungsten lamp does not have time to go out in the few milliseconds off time so you are delivering what is virtually a smoothly variable ac voltage which gives a steady brightness on the lamp and a steady indication on your ammeter. In both the above control modes the control circuit can watch the line voltage and adjust the SCR on-time continuously to achieve a constant power in the load regardless of severe fluctuations of line voltage. These are not now transmitted to the heater thus relieving the temperature controller of the job of correcting for temperature changes.