TO THE COMMITTEE OX ELECTRICAL STANDARDS. 205 



convenient, as exhibiting separately the part of the induction due to the 

 ether and the part due to a material medium. 



The customary convention of further denoting (//. — /'n)//"o by the 

 symbol 4t,,-, and then christening kH as the magnetisation I, is likewise 

 convenient. With this definition k is a pure number, and I is a gauss- 

 gradient or field-intensity. Another, but on the whole less satisfactory, 

 definition, viz., the omission of /(„ from the denominator, would make k of the 

 same dimension as ^/, and I an induction-density. 



The pull between two parallel magnetised surfaces of area A is ^ABH 

 -?-4n-, that is to say, NH/8-, and is therefore measured in webers multiplied 

 by the gauss-gradient, or in joules per centimetre. But to maintain an 

 induction density B in air requires a gauss-gradient B//in, hence we might 

 write the pull across an air-gap as N--^-S-//oA. If the induction-density 

 across an air-gap is expressed in microwebers per square centimetre the 

 tension there comes out in units of which 2,500 would make an atmo- 

 sphere ; or, roughl}', in pounds per square foot. 



As for the strength of a magnetic pole— a quantity which, though 

 fundamental in one sense, is seldom really dealt with — it will naturally be 

 expressed in ergs per gauss, or in joules per gauss if it is very strong. 



Mr. Baily's chief practical suggestions are first that a special unit of 

 permeance, other than the henry, is desirable; and next that the 47r/'10 

 had best be thrown on to the /<, so as to keep the gauss equal to one 

 ampere-turn. The fact that the inductivity of air will then cease to appear 

 in its artificial garb of unity may even be regarded as a positive advantage, 

 because its existence will then be less likely to be ignored. But I 

 much^ fear that the ampere-turn as unit of gaussage, so near the c.g.s. 

 unit in size and yet not equal to it, will be awkward and may lead to 

 mistakes. O. J. L. 



University College, Liverpool: Janvarij L5, 1895. 



Dear Professor Lodge,— In reference to the sizes of the magnetic units 

 proposed by you, I find that the weber 10« c.g.s. would only be used in 

 fractions. The largest dynamo that I know of has a magnetic flux, or, as 

 you propose to call it, an induction, of -5 weber. From this the value will 

 go down to about -01 in small motors. These figures are, however, by no 

 means inconvenient. 



Transformers will be rather smaller. In these the weber-turn is a con- 

 venient size and an interesting quantity, as it is given by \ of the mean volts 

 per cycle, or, more accurately expressed, mean volts per unit frequency -^'^. 

 Its numerical value will lie between, say, \ and 50, according to the volts 

 and the frequency ; but it gives no indication of the size of the transformer. 



The henry, 10^ c.g.s., is also large. The inductance of choking coils 

 would m general be fractions of a henry. The inductance of the winding 

 of a transformer has no very important meaning, but it has a conve*^ 

 nient size._ Measuring it as mean volts per tmit frequency -four times 

 the open circuit current in amperes, the inductance of the primary coil 

 on a 2 IP closed magnetic circuit 1,000-volt transformer would be about 

 40 henrys. 



The inductance of pairs of cables would run from 100 to 1,000 micro- 

 henrys per kilometre, but the value would vary with the arrangement. 



The induction per unit area is good ; having a value in practical work 

 from 1,000 to 20,000 c.g.s. units, it is given by 10 to 200 microwebers 

 per sq. cm. 



