Radio Terminology

Radio Record, Volume III, Issue 10, 20 September 1929, Page 19

Radio Terminology

Standardisation’ Necessary WITH the increasing popularity of the A.C. sets and the consequent ever-increasing number of resistances, there is some necessity for a. standardisation of terminology. For in- | stance, a tapped resistance may mean a resistance placed across the output of an eliminator to break down the | voltage, a small resistance placed across a filament supply to provide a centre tap, or a potentiometer. This latter term is in itself very confusing. A potentiometer is a piece of apparatus consisting of a dry cell, a1 accumulator, switching devices, a high resistance wire several metres in. length, and a variable tap. It is used for determining differences i. potential, and so resistance, hence the derivation of the name-~potentio-meter, a meter to measure potential difference. The term has for some reason or other been applied to a final resistance with a variable tap, and as this potential divider, as it should be called, now almost universally receives the name of "potentiometer," we shall in future refer to it as such. It is, then, a resistance with three terminals. The two outside may be regarded as being the ends of a fixed resistance of the value gf a potentiometer. The centre tap is variable, so that at any time any amount of ‘resistance may be placed between this tap and one of the others. A centre-tapped resistance is really a potentiometer with the moving arm fixed at the centre point. A fixed resistance ‘will indicate an untapped resistance that cannot be varied. By a "tapped resistance" will be implied a high value resistance placed across the output of an eliminator to break down the voltage. It is sometimes referred to as a "voltage divider" or a "potential divider." A grid leak is a highvalue’ fixed resistance that will pass very little current. and is suitable only |

bi for insertion in the grid lead of the . valve. A rheostat is a very indefinite name applied to types of variable re- — sistances. Its use in A.C. terminology serves only to confuse matters. We, shall henceforth refer to it as a variable resistance. In a recent article by "Megohm" (the ; anode bend: detector) the term "poten- | tiometer" ‘was altered to read "potential divider,’ which, although more correct, is nevertheless rather misleading, as any tapped resistance is a potential divider. :