psychrometer (meteorological instrument). An instrument used to measure the amount of water vapor in the air. Two thermometers are used to measure the air temperature. One thermometer has a cloth wick around its sensitive end, and this wick is dipped in water and air is blown across it.
The difference in temperature shown on the wet-bulb thermometer and the dry-bulb thermometer is an indication of the amount of cooling done by the evaporating water. The amount of cooling is inversely proportional to the amount of water vapor in the air.
P-type semiconductor material. A semiconductor material, either silicon or germanium, that has been doped with a few parts per million of an impurity atom having three electrons in its valence shell.
Boron, aluminum, gallium, and indium are elements used to dope silicon and germanium to make P-type material.
public aircraft. 14 CFR Part 1: “Aircraft used only in the service of a government, or a political subdivision. It does not include any government-owned aircraft engaged in carrying persons or property for commercial purposes.”
pulley (aircraft control system component). A grooved wheel around which a steel control cable passes. Pulleys are used to change the direction of movement of a control cable.
pull test (aircraft fabric test). A test to measure the strength of the fabric used to cover an aircraft. A strip of fabric exactly one inch wide is cut from the aircraft and all of the dope removed from it. This strip is put into a tensile tester and pulled until it breaks. The number of pounds of pull required to break the strip is the strength of the fabric in pounds per inch.
pulsating direct current. A flow of electrons that varies in its rate of flow, but does not change its direction of flow. A rectifier changes alternating current into pulsating direct current.
pulse (electrical energy). A sudden change in voltage in an electrical circuit, lasting only a short period of time. A pulse can be either an increase or decrease in the voltage, and can be used as a bit of information transmitted in the circuit.
pulse (radar technology). A brief burst of electromagnetic radiation emitted by a radar. A pulse has a very short time duration.
pulse amplifier. A wide-band electrical amplifier used to increase the voltage of a square wave of alternating current or pulsating direct current. An effective pulse amplifier does not change the waveform of square-wave pulses of energy.
pulse counter. An electronic circuit that measures the number of pulses of electrical energy it receives in a given interval of time.
pulse-echo method of ultrasonic inspection. A form of nondestructive inspection used to detect the presence of internal damage or faults in a piece of aircraft structure. Pulses of ultrasonic energy are introduced into the material being inspected, and the time required for these pulses to travel through the material and return to the transducer is measured.
If there is a fault in the material, the pulses will bounce off it, rather than traveling all the way to the rear surface of the material. This short travel time indicates the presence of a fault.
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