10. The field-strength expressed in decibels above 1 microvolt per metre will be given by the sum of: (a) the reading(s) of the attenuator(s), (b) the field-strength calibration constant (if any) appropriate to the frequency at which the measurement is being made, and (c) the reading of the indicating meter of the valve-voltmeter, if calibrated in decibels. If the result obtained is x decibels, the field strength expressed in microvolts per metre is given by the antilog to the base 10 of x/20