In X-ray sets employing an auto-transformer and variable resistance as rough and fine voltage controls, in the primary circuit, there are a number of auto-transformer tappings, which each with a suitable value of resistance, will give the correct reading on the voltmeter. Though the tube current is adjusted to its correct reading in the normal manner in every case, the output quality and quantity will alter with transformer tapping. The effect has been investigated on several sets and is also discussed from a theoretical point of view. Recommendations are made for minimizing the effect.