Vibrating reed meters
One of the most basic forms of frequency meter is the vibrating reed meter or tuned reed meter. This consists of an electromagnet coil carrying the signal positioned near the end of a tuned metal reed or tuning fork-type arrangement. As the signal travels through the coil it creates a magnetic field with the sample frequency, which pushes and pulls on the reed, or a small piece of metal or a magnet connected to it. The reed is shaped to vibrate at a particular frequency, and if the signal in the magnet is close to it, it will begin to vibrate. Multiple reeds can be positioned on a single electromagnet through various mechanical connections and the frequency of the signal can be determined by seeing which reed is vibrating the most.[2]
Similar systems, "reed receivers" were also used in early radio control systems; when the reed vibrated with enough amplitude it would cause an electrical contact to close and actuate the controls.[3]
Moving needle systems
More advanced systems were of the deflection type, ordinarily used for measuring low frequencies but capable of being used for frequencies as high as 900 Hz. There are two common types, the BTH resonance frequency meter and the Weston frequency meter. Both use electrical resonance to create a magnetic field to move a pointer, differing in their exact construction.
The BTH meter, named for the British Thomson-Houston heavy industrial firm, consists of a magnet coil connected to the input signal. Running through the center of the magnet is an iron core which extends past the end of the coil and is curved and tapered roughly like a sabre. At the other end of the core is a second coil that is allowed to move closer or further from the fixed input coil. This moving coil is connected to a capacitor to produce an LC circuit tuned to a particular frequency.
Because the iron core passes through the moving coil, and the core is tapered, the inductance of the LC circuit changes as the coil moves closer or further from the fixed input coil. When a signal is applied to the input coil, the moving coil sees a force toward or away from the input coil, and begins to move until the resulting resonance of the LC+core is the same frequency as the input signal. Normally the moving coil is suspended from a pivot above it, so the linear motion along the core causes the coil, and an attached pointer, to rotate over a dial.
The Weston frequency meter also uses tuned circuits, but in this case it is the relative inductance between two such circuits that creates the meter's movements. The system uses coils with open centers where the moving portion of the meter is positioned. Each coil has a partner that is electrically connected so that the resulting field between them is uniform like in a solenoid. Two such paired coils are used, arranged at right angles so that the resulting assembly looks like a hash mark, #, when viewed from the side.
One of the pairs of coils are connected to inductors and resistors while the second does not have any inductors. This causes the current in the inductor side to change as the signal frequency varies away from the selected base frequency, while the field in the second set of coils does not. This causes the currents in the two sets of loops to vary in relation to each other, and the resulting magnetic field between them as well. A small magnet inserted in the open center turns to align itself with the resulting field.
