Flyweight Coil spring Rocking shaft Drive shaft N S Dial Pointer Hairspring Drive cable from engine Drag cup Rotating magnet 10-31 Figure 10-54.  A simplified magnetic drag cup tachometer indicating  device.  Figure 10-53.  The simplified mechanism of a flyweight type mechanical tachometer.  this output. A drag cup, or drag disk link, is used to drive the  indicator as in a mechanical tachometer.  [Figure 10-55] Two  different types of generator units, distinguished by their type  of mounting system, are shown in  Figure 10-56 . The dual tachometer consists of two tachometer indicator  units housed in a single case. The indicator pointers show  simultaneously, on one or two scales, the rpm of two engines.  A dual tachometer on a helicopter often shows the rpm of  the engine and the rpm of the main rotor. A comparison of  the voltages produced by the two tach generators of this type  of helicopter indicator gives information concerning clutch  slippage. A third indication showing this slippage is sometimes  included in the helicopter tachometer. [ Figure 10-57] Some turbine engines use tachometer probes for rpm  indication, rather than a tach generator system. They provide  a great advantage in that there are no moving parts. They are  sealed units that are mounted on a flange and protrude into  the compressor section of the engine. A magnetic field is set  up inside the probe that extends through pole pieces and out  the end of the probe. A rotating gear wheel, which moves  at the same speed as the engine compressor shaft, alters the  magnetic field flux density as it moves past the pole pieces at  close proximity. This generates voltage signals in coils inside  the probe. The amplitude of the EMF signals vary directly  with the speed of the engine.  accessory drive section of a turbine engine. As the engine turns,  so does the generator. The frequency output of the generator is  directly proportional to the speed of the engine. It is connected  via wires to a synchronous motor in the indicator that mirrors 