
The instrument requires a specific lead and thermocouple probe which may be plugged in at the front or alternatively at the back. As with all sensitive power meters the probe has a relatively small tolerance in terms of input power. It can measure (at full scale deflection) 10 microwatts to 10 milliwatts and is accurate (with appropriate probe) from 10MHz to 40 GHz. If you would like to know this in voltage terms it will give you a reading of about 0.1 on its most sensitive scale when the probe sees 7 millivolts RMS. The switch on the lower right is set for the specific probe connected to the Thermistor Mount socket. 
The maximum power reading for this 478A probe, printed in red is expressed as 30MW average power which actually means 30mW RMS. The black ring is part of the Nconnector input socket. For some very odd reason Hewlett Packard designers have used a capital M which normally means "Mega" instead of lower case m which is "milli". They also inadvertently used upper case DBM when they should have used "dBm". The documentation accompanying the equipment and written by more knowledgeable Technical Authors is correct. The probe is calibrated on the reverse side from 0.01GHz to 10GHz, showing a curve from which a correction factor can be selected on the front panel. In case you're not familiar with dBs the designers calibrated the meter and switch in both dBm and mW. 
Above is a picture showing the construction of the instrument. The big space is for a NiCad battery. Just as British made stuff is obviously British, USmade stuff is obviously American, often using duralumin metalwork and always with coarsethreaded screws. Now a point on practical measurements. You'll see that the power meter has a range switch and a meter scale marked in dBm and mW. These scales are equivalent so 0dBm = 1milliwatt. As the probe efficiency is fequency dependent you can check the curve on the back of the probe and set the centre knob to correspond with the frequency with which you're working. Now, for power readings to be fully meaningful, you need to know the resistances associated with the scale markings. In this meter the input impedance is 50 ohms so 0dBm means 1 milliwatt dissipated in 50 ohms and that means there's should be a voltage, V across the 50 ohm input (where V=square root of the product 50 x 1mW) of 0.2236 volt to give you a display of 0dBm. As we're using RMS or root mean square measurements this means that a little under a quarter of a volt RMS present across the load resistor makes the pointer show a reading of 0dBm. Say we wish to connect a typical signal generator to the power meter with a coax cable. The signal generator will have an output impedance which is likely to be about 50 ohms. Setting the output level to 0dBm on the generator places a voltage of 0.2236 at its output socket. If we measure this voltage with a suitable high impedance voltmeter you should see a little under quarter of a volt AC, however if we now connect the power meter to this socket we place 50 ohms across the output. As power depends on voltage and current (Power=volts x amps) we have to consider any changes in current flow. If the generator output has a 50 ohm matching resistor in series with its output connector the current will pass through this and the 50 ohm input resistance of the power meter, thus half the output power will be dissipated in the generator and half in the power meter. Similarly if the output resistance of the generator is shunting its output the output voltage will now see 25 ohms and the power will be split 50/50 between the two 50 ohm impedances. This is really annoying. One sees the generator output indicated as 0dBm and you see 3dBm displayed on the power meter scale. To get around this you can use a series resistor between the generator and the power meter (the resistor needs to be reactancefree at the frequency being used so you should not use a wirewound type or a carbon film type using a circular track). If you calculate the attenuation due to the resistor you can take account of this in your measurements. If you place 450 ohms between the generator and power meter you increase the load from 50 ohms to 500 ohms. A tenth of the power indicated at the signal generator is being passed to the power meter (this is 10dB) so a reading of 0dBm on the power meter means you have +10dBm registered on the generator. This is actually an approximation because the generator actually sees a 45 ohm load instead of 50 ohms. The output voltage is likely to be 90% of that indicated so the power will be less than indicated. Increase the attenuation to 20dB by using a larger series resistor 1531 ohms and you get a big improvement or 97% of the indicated power at the generator output. A larger series resistor of 4950 ohms gives you a load of 49.5 ohms which is even better if you can tolerate an attenuation of 30dB. It's important to know lots of seemingly minor points about a measuring instrument if you're interested in really accurate numbers. For example, if you connect a voltmeter to the terminal on the rear of this power meter to see what's driving the display the operating manual says you need to make adjustments to the readings if your voltmeter has an input impedance of less than 10 Mohms (that's 10 Megohms not 10 milliohms). 