The bolometer was used with a Wheatstone bridge for measuring the r.f power. A preliminary adjustment, carried out with r.f power present, set the d.c power to a level which caused the bridge to be balanced. R.f power was then applied, and the d.c supply to the bridge was again adjusted until the bridge balanced. The r.f power was taken as the difference between the d.c power supplied to the Bolometer in the two cases, i.e one quarter of the difference between the d.c powers supplied to the bridge.