Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/305

the absence of electrification. For instance, if we have two charged bodies $A$ and $B$ we may use the method described in Chapter I to determine which body has the greater charge. Let the body $A$ be carried by an insulating support into the interior of an insulated closed vessel $C$. Let $C$ be connected to earth and again insulated. There will then be no external electrification on $C$. Now let $A$ be removed, and $B$ introduced into the interior of $C$, and the electrification of $C$ tested by an electroscope. If the charge of $B$ is equal to that of $A$ there will be no electrification, but if it is greater or less there will be electrification of the same kind as that of $B$, or the opposite kind.

Methods of this kind, in which the thing to be observed is the non-existence of some phenomenon, are called null or zero methods. They require only an instrument capable of detecting the existence of the phenomenon.

In another class of instruments for the registration of phenomena the instrument may be depended upon to give always the same indication for the same value of the quantity to be registered, but the readings of the scale of the instrument are not proportional to the values of the quantity, and the relation between these readings and the corresponding value is unknown, except that the one is some continuous function of the other. Several electrometers depending on the mutual repulsion of parts of the instrument which are similarly electrified are of this class. The use of such instruments is to register phenomena, not to measure them. Instead of the true values of the quantity to be measured, a series of numbers is obtained, which may be used afterwards to determine these values when the scale of the instrument has been properly investigated and tabulated.

In a still higher class of instruments the scale readings are proportional to the quantity to be measured, so that all that is required for the complete measurement of the quantity is a know ledge of the coefficient by which the scale readings must be multiplied to obtain the true value of the quantity.

Instruments so constructed that they contain within themselves the means of independently determining the true values of quantities are called Absolute Instruments.

Coulomb's Torsion Balance.

215.] A great number of the experiments by which Coulomb