TO THE COMMITTEK ON KLKCTIUCAL STANDAItDS. 199 



of variation gives the voltage induced in an electric circuit surrounding it 

 once. It is proposed that its practical unit be called a ' weber/ and be 

 defined as equal to 10"^ c.g.s. lines (or unit tubes) of induction. 



(Denote the quantity for the present by N, and its density by B.) 



Summary of known facts.— e being the intensity of electric force, or 

 the slope of electric potential, or the volt-gradient at a point ; the cir- 

 cuitation of e=the induced EMF in a closed circuit=the rate of chan^^e 

 of induction through it ; 

 or, cycle | eds='E= - X= - J J i5(?S ; 



or, at any point of space, 



B=r— cur] (,'=— Yve. 



Through a simple closed electric circuit of constant resistance, 



N= - |Ef7<= \'RCdt=nq,. 



(10) The total induction through any area may be practically measured 

 by suddenly surrounding it with a closed wire circuit of ii turns connect- 

 ing the terminals of a ballistic galvanometei-, and measuring the quantity 

 of electricity thereby impelled through the galvanometer. The induction 

 is equal to the quantity so impelled, multiplied by Hh the resistance of 

 the circuit. If the quantity is one coulomb, and the resistance one ohm, 

 the induction is l/';(th of a weber. 



Or, otherwise, if the induction through any boundary changes at 

 the rate of one weber per second, the EMF excited in that boundary is 

 1 volt. 



In the case of a spiral wire circuit through wliich induction is varying at 

 the rate of a weber per second, one volt is excited in each turn of the wire. 



(11) Another tx\oAq of measuring the total induction through an area 

 is to surround that area with a movable electric circuit of n turns of wire 

 conveying a known curi'ent, and to measure the potential (or mechanical) 

 energy of the circuit under those conditions. The induction is equal to 

 the potential energy of the circuit divided by ')i times the current circu- 

 lating in each turn of wire. 



Or, if the induction through a simple circuit carrying one ampere is 

 one weber, the potential energy of the circuit is one joule. 



Derived quantities. 



(12) Induction- density, or B, may be expressed as so many webersper 

 unit area ; say per square centimetre or per square inch, or whatever is 

 preferred for practical purjioses. 



For instance, the earth's horizontal induction-density at some place is 

 •18 c.g.s. unit, = -18 x 10~** weber per square centimetre. =18 micro webers 

 per square metre. 



(13) The inductivity (fi), or absolute permeability of a medium at any 

 point under specified circumstances, is the ratio of B to H at that point, 

 and under those circumstances. In many substances this ratio is far from 

 constant. [It may be expressed in terms of henry s or other units of per- 

 meance per unit length (see below), instead of in c.g.s. units, if convenient. 

 For example, the inductivity of air is T-ffu^h of a microhenry per centimetre 



