0° Transmitter A N S N S N S N S B C D 25° 50° Indicator Rotor shaft Brushes Contact arm Resistance winding Pointer Instrument scale Permanent magnet Figure 10-47.  A schematic of a DC selsyn synchro remote indicating  system.  DC Selsyn Systems 10-27 Referring to  Figure 10-47 , note that the resistance winding  of the transmitter is tapped off in three fixed places, usually  120° apart. These taps distribute current through the toroidial  windings of the indicator motor. When current flows through  these windings, a magnetic field is created. Like all magnetic  fields, a definite north and south direction to the field exists. As the transmitter rotor shaft is turned, the voltage-supplying  contact arm moves. Because it contacts the transmitter  resistance winding in different positions, the resistance  between the supply arm and the various tapoffs changes. This  causes the voltage flowing through the tapoffs to change as  the resistance of sections of the winding become longer or  shorter. The result is that varied current is sent via the tapoffs  to the three windings in the indicator motor.  Landing gear contain mechanical devices that lock the gear  up, called an up-lock, or down, called a down-lock. When  the DC selsyn system is used to indicate the position of the  landing gear, the indicator can also show that the up-lock  or down-lock is engaged. This is done by again varying  the current flowing through the indicator’s coils. Switches  located on the actual locking devices close when the locks  engage. Current from the selsyn system described above  flows through the switch and a small additional circuit. The  circuit adds an additional resistor to one of the transmitter  winding sections created by the rotor arm and a tapoff. This  changes the total resistance of that section. The result is a  change in the current flowing through one of the indicator’s  motor coils. This, in turn, changes the magnetic field around  that coil. Therefore, the combined magnetic field created by  all three motor coils is also affected, causing a shift in the  direction of the indicator’s magnetic field. The permanent  magnet and pointer align with the new direction and shift to  the locked position on the indicator dial.  Figure 10-48  shows  a simplified diagram of a lock switch in a three-wire selsyn  system and an indicator dial. On aircraft with direct current (DC) electrical systems, the  DC selsyn system is widely used. As mentioned, the selsyn  system consists of a transmitter, an indicator, and connecting  wires. The transmitter consists of a circular resistance  winding and a rotatable contact arm. The rotatable contact  arm turns on a shaft in the center of the resistance winding.  The two ends of the arm are brushes and always touch the  winding on opposite sides.  [Figure 10-47]  On position  indicating systems, the shaft to which the contact arm is  fastened protrudes through the end of transmitter housing and  is attached to the unit whose position is to be transmitted (e.g.,  flaps, landing gear). The transmitter is often connected to the  moving unit through a mechanical linkage. As the unit moves,  it causes the transmitter shaft to turn. The arm is turned so  that voltage is applied through the brushes to any two points  around the circumference of the resistance winding. The rotor  shaft of DC selsyn systems, measuring other kinds of data,  operates the same way, but may not protrude outside of the  housing. The sensing device, which imparts rotary motion  to the shaft, could be located inside the transmitter housing. The resultant magnetic field created by current flowing  through the indicator coils changes as each receives varied  current from the tapoffs. The direction of the magnetic field  also changes. Thus, the direction of the magnetic field across  the indicating element corresponds in position to the moving  arm in the transmitter. A permanent magnet is attached to  the centered rotor shaft in the indicator, as is the indicator  pointer. The magnet aligns itself with the direction of the  magnetic field and the pointer does as well. Whenever the  magnetic field changes direction, the permanent magnet and  pointer realign with the new position of the field. Thus, the  position of the aircraft device is indicated.  These systems are similar in construction, and all operate  by exploiting the consistent relationship between electricity  and magnetism. The fact that electricity can be used to create  magnetic fields that have definite direction, and that magnetic  fields can interact with magnets and other electromagnetic  fields, is the basis of their operation.  a certain parameter on the aircraft. Most position-indicating  instruments are designed around a synchro system, such as  the flap position indicator. Fluid pressure indicators also  commonly use synchro systems. Synchro systems are used as  remote position indicators for landing gear, autopilot systems,  radar, and many other remote-indicating applications. The  most common types of synchro system are the autosyn,  selsyn, and magnesyn synchro systems. 