373Microwave Measurements
From Joules–Lenz’s law
Q = 0.24 I
2
R t = 0.24 Pt (10.20b)
Thus, Q = W ΔT = 0.24 Pt (10.20c)
P = W ΔT/0.24 t = 4.17 W ΔT/t (10.20d)
where P is the unknown power (to be measured) and t is the measurement
time. The volume W is determined as the difference between readings of
total ow meter at the beginning and at the end of the observation.
The temperature difference is measured by thermometers or by thermo-
couples after a steady state is reached. With thermocouples connected in
series opposition, the micro-ammeter may be calibrated in unit of ΔT. If, in
addition, the rate of ow of water (u = W/t) is maintained constant, the cali-
bration may be directly obtained in the units of power (P = 4.1 u ΔT = a