ELECTRICAL UNITS OF MEASUREMENT. 123 



convenient. But when you go farther with the 

 practical system, and take anything that involves 

 a magnetic pole or a magnetic field, you get lost 

 in the trouble of adopting the earth's quadrant 

 as unit of length, and deviation from C.G.S. ceases 

 to be convenient. Return then to C.G.S. pure 

 and simple. 



I spoke of the resistance of an ohm being 

 measured in terms of a velocity. I should like 

 to explain this in a few words. Imagine a 

 mouse-mill set with its axis vertical. Put a pair 

 of brushes at the tops and bottoms of the bars ; 

 put the brushes in the magnetic north and south 

 plane through the axis, and set the mouse-mill 

 to spin at any rate you please. Take a galvano- 

 meter like a tangent galvanometer, but with only 

 an arc of wire equal in length to the radius an 

 arc subtending an angle equal to about 57'3 

 having its ends on the same level, whether above 

 or below the level of the needle, and electrodes 

 perpendicular to the plane of the arc connected 

 with the brushes. The mouse-mill must be placed 

 so far from the galvanometer, as not sensibly to 



