17 



measurement in terms of resistance and time. The standard is some form of 

 condenser consisting of two sets of metal plates separated by a dielectric. 

 The condenser should be surrounded by a metal shield connected to one set 

 of plates rendering the capacity independent of the surroundings. An ideal 

 condenser would have a constant capacity under all circumstances, with zero 

 resistance in its leads and plates, and no absorption in the dielectric. Actual 

 condensers vary with the temperature, atmospheric pressure, and the voltage, 

 frequency, and time of charge and discharge. A well-constructed air con- 

 denser with heavy metal plates and suitable insulating supports is practically 

 free from these effects and is used as a standard of capacity. 



Practically, air-condenser plates must be separated by 1 mm or more and so 

 cannot be of great capacity. The more the capacity is increased by approaching 

 the plates, the less the mechanical stability and the less constant the capacity. 

 Condensers of great capacity use solid dielectrics, preferably mica sheets with 

 conducting plates of tinfoil. At constant temperature the best mica condensers 

 are excellent standards. The dielectric absorption is small but not quite zero, 

 so that the capacity of these standards found varies with different methods of 

 measurement, so for accurate results care must be taken. 



INDUCTANCE 



The henry, the unit of self-inductance and also the unit of mutual inductance, 

 is the inductance in a circuit when the electromotive force induced in this 

 circuit is 1 volt, while the inducing current varies at the rate of 1 ampere per 

 second. 



Inductance standards. — Inductance standards are measured in interna- 

 tional units in terms of resistance and time or resistance and capacity by alter- 

 nate-current bridge methods. Inductances calculated from dimensions are in 

 absolute electromagnetic units. The ratio of the international to the absolute 

 henry is the same as the ratio of the corresponding ohms. 



Since inductance is measured in terms of capacity and resistance by the 

 bridge method about as simply and as conveniently as by comparison with 

 standard inductances, it is not necessary to maintain standard inductances. 

 Thev are however of value in magnetic, alternating-current, and absolute 

 electrical measurements. A standard inductance is a circuit so wound that 

 when used in a circuit it adds a definite amount of inductance. It must have 

 either such a form or so great an inductance that the mutual inductance of the 

 rest of the circuit upon it may be negligible. It usually is a wire coil wound all 

 in the same direction to make self-induction a maximum. A standard, the in- 

 ductance of which may be calculated from its dimensions, should be a single 

 layer coil of very simple geometrical form. Standards of very small inductance, 

 calculable from their dimensions, are of some simple device, such as a pair of 

 parallel wires or a single turn of wire. With such standards great care must 

 be used that the mutual inductance upon them of the leads and other parts of 

 the circuit is negligible. Any inductance standard should be separated by long 

 leads from the measuring bridge or other apparatus. It must be wound so that 

 the distributed capacity between its turns is negligible ; otherwise the apparent 

 inductance will vary with the frequency. 



POWER AND ENERGY 



Power and energy, although mechanical and not primarily electrical quanti- 

 ties, are measurable with greater precision by electrical methods than in any 



SMITHSONIAN PHYSICAL TABLES 



