Technologies of a long forgotten era...

Most of the technologies described below had clever tricks to be unaffected by power supply fluctuations, as a simple single moving coil indicator would be, however, the trade-off is complexity.


Quadrature phase motors are used in various indicators, such as the fuel tank indicators. They are used to operate the pointer or display, but in conjunction with some form of feedback to an amplifier that drives the motor, this way the pointer or display can be positioned correctly using the motor.

Most of the quadrature phase motors in the Lightning indicators are fed with a constant reference 400Hz AC Voltage to a Reference coil, and a Control coil is fed a 400Hz AC Voltage but 90∘ out of phase. The amplitude of the control signal will affect the speed of the motor, and the phase rotation (90∘ leading or lagging the reference signal) will determine the direction of rotation.


Engine speed of the Lightning is measured by a small electrical generator attached to an ancillary shaft on the side of the Avon engine. It produces a 3∅ AC voltage with a frequency dependant on engine speed (N1 RPM).

The indicator contains a 3∅ synchronous motor, and are connected directly to the generator with no external power supply. Being a synchronous type, the motor rotates in sync with the generator, and hence engine speed (N1).

Inside the indicator, the motor rotates a magnet, which acts upon a copper drag-cup, based on the principal of eddy currents, the cup is rotated against a spring, the faster the motor, the further the cup compresses the spring. The cup position actuates the mechanism to operate the pointers.


Most commonly used for position feedback of control surfaces etc, the Desynn transmitter contains a special completely circular potentiometer with no ends, but three equally spaced electrical tappings. The Desynn potentiometer has two wipers spaced 180∘ apart, one connected to ground, the other to 28Vdc.

The Desynn transmitter therefore produces three voltages relative to shaft angular position.

The Desynn indicators contain three coils, connected in Delta arrangement to the three outputs of the transmitter, the voltages of the three connections, and hence current flows, induce three diffrent magnetic fields which act upon the pointer position.

As a Desynn transmitter can theoretically operate through a full 360∘ (only limited mechanically by design), so too can the indicator, and the two will always be perfectly in sync with each other.

The Desynn system is not affected by fluctuations in power supply voltage, as the ratio of three transmitter voltages (and hence curents and field strengths) remain constant, and hence pointer position will remain true to the transmitter angle.


A ratiometer works on the principal of positioning the pointer depending on the difference between the field strength of two magnetic coils within the indicator, ie: the ratio of their field strengths. Physically one coil is static in the housing, and the other is a moving coil on the pointer shaft.

The coils are usually connected to a transmitter (temperature / pressure / position etc..) whereby a potentiometer is acted upon by the mechanical sensing element. The wiper of the potentiometer is usually connected to ground, and the ends of the track are connected to the two coils of the ratiometer indicator.

The other ends of the two coils are usually commoned together and connected to +28Vdc, thus completing the circuit. The circuit and principal do resemble those of a Wheatstone Bridge.

As the potentiometer wiper of the transmitter is moved, the currents in the two indicator coils are biased ond way or the other (as one increases, the other decreases), affecting the indicator coil field strengths, moving the pointer according to the ratio of the field strengths.

The advantage that a ratiometer has over a standard single moving coil indicator is that changes in power supply voltage do not affect its operation. As a ratiometer displays the difference between two currents, if the power supply voltage falls, both currents will fall by the same proportion, so the ratio remains unchanged, and hence, pointer position remains true.


Used within some indicators for pointer / display position feedback. Potentiometers are electrically simple devices, just a variable resistor, but the types used in these indicators are of very high mechanical build quality for reliability.


As with potentiometers, synchro control transmitters are used for pointer position feedback in some indicators.

They are an AC device, effectively a transformer with one primary coil, and three secondary coils. The primar, or reference coil is supplied with 400Hz AC signal, and the secondaries produce 3 ACsignnals. The voltage levels of these three secondary signals vary depending on shaft angle.


Often referred to as a "dolls eye" these indictors generally have a rotating element which can show two differently painted sides through a window.

Reasons for using a magnetic indicator over an illuminated indicator would include reliability, as fillament lamps expire, and due to this they are often installed in pairs in case of bulb failure. Also sunlight will not obscure the indicator as an happen with illuminated indicators.

They operate by passing a current through a coil, the magnetic field generated acts upon the indicator, rotating it.