INSTRUMENTS Measurements of capacity and inductance are generally made by comparison with fixed or variable standard condensers or in ductances. Condensers are of various forms, fixed condensers of high capacity from o•ooi to many microfarads, variable vane condensers of low capacity as are used in radio, standard air con densers, and high voltage glass condensers of the Leyden jar type.
High capacity fixed con densers are generally made of rectangular sheets of tinfoil be tween somewhat larger sheets of paper impregnated with paraffin wax. Each tinfoil sheet is pro vided with a projection or lug at one side, and alternate sheets have their lugs projecting in dif ferent places so that they can be connected together. Where a sufficient number of tinfoil and paper sheets have been laid to gether to give the required capacity the whole condenser is pressed between hot plates so as to form a solid block when cold, the alternate lugs are soldered together and connected to the ter minals, and the whole block inserted in a wooden or metal case which is filled up solid with paraffin wax. For highly accurate standards thin sheets of the best ruby mica are substituted for the paper, and these sheets are frequently silvered on one side, the silvering being removed around the edges. A number of such paper or mica condensers are frequently included in a single case and can be used singly or in parallel, and dial condensers resem bling resistance boxes, enabling the capacity to be adjusted from o•ooi to i•III microfarads are now commonly used.
Variable vane condensers consist of two sets of thin metal plates, usually of semicircular form, but of different diameters. The larger or fixed plates are built up on a frame so as to be accurately parallel to one another and equally spaced, while the smaller are threaded on a spindle with distance pieces between them so as also to be parallel and similarly spaced. The spindle is mounted in the frame so that the movable plates or vanes can turn between the fixed plates without touching them, and the two systems of fixed and moving plates are insulated from one another. An insu lating handle provided with a pointer or a rotating dial enables the spindle to be turned, and the capacity varies continuously, from a low minimum value when the moving vanes are outside the fixed plates, to a maximum when they are completely inside them. By inserting the whole system in a case filled with oil both the capacity and maximum safe P.D. of the condenser may be considerably in creased. The vanes are sometimes made of various forms in order to obtain a "square law" variation of capacity with angle, or a uniform frequency scale, etc.
Standard air condensers are only used for fundamental measure ments and usually consist of a number of concentric accurately turned and measured cylinders on an insulating base so that their capacity can be calculated from the dimensions.
The most simple method of comparing the capacity of con densers is by the direct deflection method using a ballistic gal vanometer (q.v.). If a condenser of capacity C is connected to a battery of e.m.f. E, it suddenly becomes charged with a quantity Q = CE, so that if two condensers of capacities and are con nected successively to the same battery = and = thus Q2 = C2 = D2 ' where and D2 are the corresponding Ql swings of the ballistic galvanometer.
Inductance standards may be either fixed or variable, and of self or mutual inductance. Fixed self-inductance standards are simply coils wound on rigid insulating bobbins, so as to have con stant conformation. Marble is used for the most accurate, and carefully seasoned paraffined teak or boxwood for the ordinary standards. The first accurate variable inductance standard or variometer was that of Ayrton and Perry, and consisted of two coils each of about ten inches diameter wound in single layers on circular wooden or ebonite formers curved so as to be parts of spheres and to enable one to fit as closely as possible inside the other. The inner coil was mounted on a spindle passing through the outer coil and having a pointer reading on a large dial at the top. The two coils were connected in series through flexible leads, and when they were parallel but the current passed round them in opposite directions their inductance was very low as the magnetic effects of the two coils almost neutralized each other, while on turning the inner coil round, the magnetic field (and therefore the inductance) increased steadily to a maximum when it was again parallel to the fixed coil. By careful construction a fold range of inductance was secured.
Variable mutual inductance standards are, however, to be preferred, as they can easily be made of very long range. The best known is that of Campbell who em ploys three circular coils on marble form ers and with their planes parallel. The centre coil is mounted on a spindle at its edge so that it can be swung like a vane between the two fixed coils, and this spindle is provided with a handle and pointer travelling over a scale calibrated in microhenries. The winding on the fixed coils is of ten insulated wires stranded to gether so as to have equal magnetic effect. In addition there are a number of fixed bobbins with primary and secondary wind ings which can be thrown into the circuit when this is desired.
Capacity and inductance measurements are now most com monly made by bridge methods using alternating current and telephones or vibration galvanometers, and such bridges allow of condensers, self, or mutual inductances to be compared in any combination. The best known forms of such bridges are those of de Sauty, Maxwell, Campbell, Drysdale and Schering.