ELECTRICAL MEASUREMENTS. 281 



upon the strength of the current, the relative positions of 

 the current and magnet, and the length of the conductor 

 which is within acting range of the magnet. The effect 

 also depends upon the strength of the magnet itself ; but 

 we may consider that we always deal with a magnet of 

 unit strength, and may therefore leave it out of con- 

 sideration now. The simplest rule to adopt to obtain a 

 standard of measurement founded upon this action, is 

 then to say that we will call that current a current 

 of unit strength, of which a unit length placed at 

 unit distance from any given point causes unit magnetic 

 force at that point. If you consider that statement you 

 will see at once some of the conditions that are necessary 

 in order to apply a system of measurement founded upon 

 the principle we are dealing with. If we are to have a 

 given length of a conductor, all of it at some definite 

 distance from a particular point, it follows at once that 

 the conductor must be bent into a circular arc. If we 

 have a line and every point 'of this line is to be at the same 

 distance from some particular point, the line must not be 

 straight, but it must be curved into the arc of a circle of 

 which this point is the centre. In order not to complicate 

 the matter by using unusual units, I will suppose we 

 take a foot as the unit of length. Take a wire a foot long 

 and bend it into the arc of a circle of one foot radius, so 

 that each point of it is at one foot distance from the centre. 

 Then suppose that we have an electric current flowing 

 along this wire ; this current will cause any magnet near it 

 to be acted upon by a force in one direction or the other, 

 and suppose that we have a magnet at the centre point. 

 If the magnetic force exerted at this particular point by the 

 current is the force one, say a force equal to the weight of 

 one grain, the strength of the current which flows along 

 the conductor is the standard of measurement it is repre- 

 sented by unity. And if the force exerted here is 2, 

 the strength of the current also is represented by 2 ; or 

 if the force at the centre is 100, the strength of the current 

 is 100. So that we take as the standard of measurement 

 of the strength of the current, the magnetic force exerted 

 at the centre of a circle of unit radius by unit length of 

 the current bent round the circumference of the circle. 

 Practically, this precise arrangement would not be con- 



