198 KEPOKT~1805. 



First quantity to he named. 



(4) The iirst thing requiring a name is this quantity magnetic 

 potential, sometimes called magneto-motive force ; a quantity spoken of and 

 measured, not inconveniently but with insufficient generality, by electrical 

 engineers as ampfere-turns. It has been proposed (by Mr. Heaviside, for 

 instance) that it be called gaussage, and that its c.g.s. unit be one gauss. 



(5) The circuital gaussage round a closed curve is 4- times the total 

 electric current through the area bounded by that curve. 



In the case of a magnetic circuit wound with wire the gaussage is 



1 j- times I ' or 1-2.jGG) the ampere-turns threading that circuit. 



A'ote. — It may be best to retain the word ' gaussage ' for the whole of 

 a closed circuit only, and to speak of the ditf'erence of magnetic potential 

 between two points as the fall of gausses or the ' gauss-fall ' from a to b. 



(6) The gaussage, or gauss-fall, in any portion a b oi a magnetic 

 cii'cuit, is measured by the change in the potential energy of a unit pole as 

 it moves from a to h by any path which involves neither the cutting of 

 magnetic layers nor the encircling of currents (a long channel being 

 imagined for its motion through solid material if necessary). Or, more 

 practically, it is measui'ed hy the induction througli a long narrow tube 

 whose ends are at a, and b respectively, divided by the permeance of that 

 tube. {Cf. Chattock on a magnetic potentiometer, Fliil. Mag., July 1887.) 



In practice, however, gaussage is frequently calculable from the 

 ampere-turns to which it is due. 



(7) Intensity of magnetic force, or H, will be naturally expi'essed as 

 gauss-fall per centimetre, or the gauss-gradient. For instance, the earth's 

 horizontal intensity at some place is •18 gauss per linear centimetre, or 

 5'4 gausses per foot. 



Hole. — H sliould not (strictly speaking) be expressed as so many lines 

 per square centimetre ; that mode of expression should be reserved for 

 induction-density B. H is the cause, and should be thought of as the slope 

 of magnetic potential, B is the effect. In a medium of so-called unit 

 permeability the two quantities are numerically equal, but they should 

 not be confounded ; any more than the slope of cleccric potential, or 

 electric-intensity (e), should be thought of as identical with current- 

 density, even in a medium of unit conductivity. 



(8) The gauss-gradient inside a long or closed magnetic solenoid of 

 length /, wound uniformly with n tarns of wire each conveying the current 

 C|, is 4 7rnC| ;7=:47r;i,C| ; where w, is the total number of turns of 

 wire (in all the layers) to the linear centimetre. 



This is the measure of H in the interior of such a solenoid, quite irre- 

 specti\e of the material with which it may happen to be filled. 



{8a) That the rotation of the plane of polarisation caused by any 

 transparent body is equal to the number of gausses between the points 

 where the ray enters and leaves the body, multiplied by the appropriate 

 specific constant of its material (sometimes called Verdet's constant) ; in 

 other words, that Verdet's constant may be expressed in degrees or radians 

 per gauss. 



Second quantity to be named. 



(9) The second quantity requiring a name is the total inductio7i in a 

 magnetic circuit, also called 'total flux,' 'total lines,' ' electro-magnetic 

 momentum,' and ' electrotonic state.' It is the quantity whose time-rate 



