APPENDIX II 

 AMPERE-TURN vs. GILBERT 



THE reader has probably been taught before that the per- 

 meability of air is equal to unity in the electromagnetic C.G.S. 

 system; silent assumption was then probably made that /*=! 

 also in the practical ampere-ohm system. The true situation 

 is, however, as follows: In any system of units whatsoever, the 

 fundamental equation $ = (fiA/l) . M holds true, being a mathe- 

 matical expression of an observed fact. Now let the quantities 

 be expressed in the ampere-ohm system, and assume the centi- 

 meter to be the unit of length. The flux is then expressed 

 either in maxwells or in webers, both of which are connected 

 with the ampere-ohm system through the volt. The natural 

 (though not the only possible) unit for the magnetomotive force 

 is one ampere-turn. Therefore, all the quantities in the fore- 

 going equation are determinate, and the value of jj. cannot be 

 prescribed or assumed, but must be determined from an actual 

 experiment, the same as the electric conductivity of a metal, 

 or the permittivity of a dielectric have to be determined. 

 Experiment shows that /*= 1.257 when the maxwell is used as the 

 unit of flux, and hence /*=1.257X10~ 8 if the flux is measured in 

 webers. 



It is possible to assume /= 1, provided that the unit of mag- 

 netomotive force is not prescribed in advance. In this case, 

 the unit of magnetomotive force, as determined from experiment, 

 comes out equal to 1/1.257 of an ampere-turn. This unit is 

 called the gilbert, and it must be understood that the permeability 

 of non-magnetic materials is equal to unity only if the magneto- 

 motive force is measured in gilberts. To the author the advan- 

 tages of such a system for practical use are more than doubtful. 

 In the first place, the gilbert is a superfluous unit, because the 

 results of calculations must after all for practical purposes be 

 converted into ampere-turns in order to specify the number of 



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