282 LECTURES TO SCIENCE TEACHERS. 



venient, and, therefore, instead of taking a wire, say one foot 

 long, and putting that partly round the circumference of a 

 circle of a foot radius we take a complete circumference ; that 

 is, if the radius of the circle is 1, the circumference 

 would be about 6*28 ; but the magnetic force at the centre 

 of the circle will be just proportional to the length of the 

 conductor. If we take one foot round a circle then unit 

 current would exert unit force ; but if we take six feet 

 round the circle then unit current in the circumference 

 would exert six times unit force; so we consider that 

 unit current is the current which, going once round a circle 

 of unit radius, exerts at the centre, not unit force, but 6 '28 

 units of force. 



This principle is adopted in all such instruments as 

 these, where we have a current carried once round or several 

 times round a circle. If I send a current round this circle 

 I shall be able to cause a deflection of the magnet at the 

 centre. You will see from what I have said timt the 

 problem we have to deal with in measuring an electric 

 current by the method I have been speaking of, is to take 

 a measured length of the current at a measured distance 

 from a definite point, and to measure the magnetic force 

 exerted at that point. The easiest way of measuring the 

 magnetic force exerted at the centre of the circle is to 

 observe the deflection from the natural position which a 

 small magnet placed at the centre undergoes. If we know 

 the magnetic force exerted at any given point due to the 

 magnetic force of the earth, then it is easy to ascertain what 

 is the force exerted on the magnet when it is deflected 

 through a measured angle from the position it would take 

 if acted upon by the force of the earth only. We have a 

 little magnet at the centre of the circle, acted upon by the 

 earth, and it takes a definite position in consequence. If 

 we act upon it also by an electric current we in general 

 displace it from the position it would take if acted on by 

 the earth only, and if we know the force exerted upon the 

 magnet by the earth, and if we know also the extent to 

 which it is deflected by an electric current, then we can 

 measure the force exerted by the electric current. In that 

 way we get to know what the force is which the current 

 exerts ; by measuring the radius of the circle, and the 

 number of times which the wire goes round, we get the 



