Alternating current ammeters or voltmeters indicate the _virtual_ values of the current or pressure respectively, that is to say, they indicate, the _square root of the mean square of a variable quantity_.
[Illustration: FIG. 2,491.--Line curve of alternating current, illustrating various current or pressure values. The virtual value, or .707 × maximum value, is the value indicated by an ammeter or voltmeter. Thus, if the maximum value of the current be 100 volts, the virtual value as indicated by an ammeter is 100 × .707 = 70.7 amperes.]
The virtual value of an alternating current or pressure _is equivalent to that of a direct current or pressure which would produce the same effect_.
For instance an alternating current of 10 virtual amperes will produce the same heating effect as 10 amperes direct current.
The relation of the virtual value of an alternating current to the other values is shown in fig. 2,491. When the current follows the sine law, the square root of the mean square, value of the sine functions is obtained by multiplying their maximum value by 1 ÷ √2̅ or .707.
[Illustration: FIG. 2,492.--Wagner tubular aluminum pointer.]
The word ~effective~ is commonly used _erroneously_ for ~virtual~, even among the best writers and the practice cannot be too strongly condemned[3].[4] The difference between the two is illustrated in Guide No. 5, page 1,013, fig. 1,237, the mechanical analogy here given may make the distinction more marked.
[3] NOTE.--I adhere to the term virtual, as it was in use before the term efficace which was recommended in 1889 by the Paris Congress to denote the _square root of mean square value_. The corresponding English adjective is _efficacious_, but some engineers mistranslate it with the word _effective_. I adhere to the term virtual mainly because effective is required in its usual meaning in kinematics to represent the resolved part of a force which acts obliquely to the line of motion, the effective force being the whole force multiplied by the cosine of the angle at which it acts with respect to the direction of motion.--_S. P. Thompson._
[4] NOTE.--The author adheres to the term _virtual_ because in mechanics the adjective _effective_ is used to denote the difference of two opposing forces; for instance, at any instant in the operation of a steam engine, _effective pressure = forward pressure - back pressure_, hence, to be consistent in nomenclature, the term effective cannot be used for the forward or virtual pressure, that is, the pressure impressed on an electric circuit.