differential coil. A coil wound in such a way that its magnetic field opposes the field produced by another coil.
differential compression check (reciprocating engine maintenance check). A maintenance check used to determine the condition of the piston rings and the valves in the cylinders of a reciprocating engine. An air pressure regulator holds a constant pressure of 80 psi on the input side of a calibrated orifice. The output side of the orifice is connected to the cylinder through an adapter screwed into a spark plug hole.
Air flowing into the cylinder, to replace that which leaks past the piston rings and valves, causes a pressure drop across the orifice. The amount of leakage is indicated by the difference in the readings of the two pressure gages on the tester.
The sound of leaking air heard at the carburetor air inlet indicates a leaking intake valve. A leaking exhaust valve is indicated by the sound being heard at the exhaust tail pipe. Worn piston rings are indicated by the sound of leaking air being heard at the crankcase breather.
differential compression tester (reciprocating engine test equipment). Test equipment consisting of an air pressure regulator, two pressure gages with a calibrated restrictor orifice between them, and a shutoff valve.
A regulated pressure of 80 psi is directed into the cylinder being tested, and the pressure drop across the orifice indicates the amount of air leaking past the piston rings or valves. A cylinder in good condition should hold at least 75% of the air supplied through the tester. With 80 psi on the input pressure gage, the cylinder pressure gage should indicate at least 60 psi.
differentially compounded electric motor. An electric motor having both series and shunt field windings. The two windings are connected in such a way that the magnetic field produced by the shunt winding opposes the field produced by the series winding.
differential pressure. A single pressure which is the difference between two opposing pressures. The resultant of a 12 psi pressure opposing a 32 psi pressure, is a 20 psi differential pressure. This is expressed as 20 psid.
differential-pressure range of aircraft cabin pressurization. The range of cabin pressurization in which the controller maintains a constant difference between the pressure of the air inside the structure and the pressure of the air outside the structure.
The amount of differential pressure allowed is determined by the strength of the aircraft structure.
differential screw. A compound screw arrangement. A differential screw changes the separation between two parts by the amount of difference in the pitch of the two screw threads.
If the coarse screw increases the space 0.050 inch for each revolution, and the fine screw decreases the space 0.0357 inch for each revolution, one revolution of the differential screw will increase the space by 0.0143 inch.

differential-voltage reverse-current relay (aircraft electrical system component). A reverse-current relay installed with a high-current aircraft generator. The reverse-current relay closes to connect the generator to the battery when the generator voltage is a specified amount higher than the voltage of the battery.