of a straight current on a magnetic pole. 213 



the funnel is open, mercury having first been poured into the 

 funnel, the mercury flows through B and soon fills G, running over 

 its edge. Between B and G the jet of mercury in the air forms 

 a continuous metallic connection so that we have a straight 

 current flowing between A and D. 



The magnet NS is formed of a thin strip of steel with two 

 needles soldered at a point nearer one end than the other. The 

 magnet is supported by the two needles resting in sockets E and F, 

 so that the magnet can only turn about a vertical axis. These 

 sockets can be easily made from pieces of glass rod. The sockets 

 are held on a stand which can be moved independently of the 

 stand carrying the glass tubes and mercury. The magnet is then 

 adjusted so that the distance of the axis of support from the 

 mercury jet is greater than its distance from one end of the magnet 

 and less than its distance from the other. The height of the 

 magnet is adjusted so that the longer arm of the magnet can pass 

 through the gap between the tubes B and G, while the shorter 

 arm never reaches as far as this gap. When the mercury flows 

 and the tube G is filled, the current passes along the straight con- 

 ductor BG. If now the magnet NS revolves once completely the 

 North pole passes round the current while the South pole describes 

 a circle not enclosing the current. The magnet can pass through 

 the conductor, breaking the current for a very short time by 

 interrupting the flow of mercury. If set revolving, the magnet 

 will therefore continue to revolve, the energy required being 

 obtained from the work done on the North pole. 



With the magnet arranged as in this experiment there is no 

 position in which it is in equilibrium, so that it will begin to 

 rotate about the vertical axis whatever its position may be when 

 the current begins to flow. 



In fig. 2, let be the section of the current which is supposed 

 to be perpendicular to the plane of the 

 paper, NS the magnet, A the axis on 

 which the magnet revolves. 



If i be the current, /x the pole- 

 strength of the magnet, the force on 

 the North pole is 



2i/j, 

 ON' 



along NP where NP is perpendicular 

 to ON 



Let iP be perpendicular to NP, 

 then the moment of the force on this 

 pole round A is 



.. AP 



