160 ELEMENTARY LESSONS ON [CHAP. nr. 



in a clockwise direction), it will repel it. If the pole be 

 thrust right into the ring, and then held still, the battery 

 will be strongly repelled, will draw itself off, float away, 

 turn round so as to present toward the S. pole of the 

 magnet its N. -seeking face, will then be attracted up, 

 and will thread itself on to the magnet up to the middle, 

 in which position as many magnetic lines of force as 

 possible cross the area of the ring. 



It can be shown also that two circuits traversed by- 

 currents attract and repel one another just as two 

 magnetic shells would do. 



It will be explained in Lesson XXVI. on Electro- 

 magnets how a piece of iron or steel can be magnetised 

 by causing a current to flow in a spiral wire round it. 



195. Strength of the Current in Magnetic 

 Measure. When a current thus acts on a magnet pole 

 near it, the force f which it exerts will be proportional 

 to the strength / of the current, and proportional also 

 to the strength m of the magnet pole, and to the length 

 / of the wire employed ; it will also vary inversely as 

 the square of the distance r from the circuit to the 

 magnet pole. Or, f IL -^- dynes. Suppose the wire 

 looped up into a circle round the. magnet pole, then 

 / = 2irr, and / = m dynes. Suppose also that the 



circle is of one centimetre radius, and that the magnet 

 pole is of strength of one unit (see Art. 125), then the 



force exerted by the current of strength i will be x i, 



or 2TZ dynes. In order, therefore, that a. current of 

 strength i should exert a force of i dynes on the unit pole, 



one must consider the current as travelling round only 



27T 



part of the circle, or round a portion of the circum- 

 ference equal in length to the radius. 



196. Unit of Current Strength. A current is 

 said to have a strength of one " absolute " unit when it 



