554 JAMES CLERK MAXWELL. 



S at rest is inexperienced the effect upon him is very 

 striking). 



If a pin be placed in the face of the wheel S, and one 

 end of a spring press against the pin, while the other end 

 is fixed to the frame of the apparatus, we have a repre- 

 sentation of a secondary coil in which the circuit is broken, 

 and a Leyden jar inserted with its coatings in connection 

 with the ends of the wire. When the motion of P is 

 changed, S will begin to move, and will deflect the spring, 

 corresponding to a current in the secondary coil charging 

 the Leyden jar. If the spring admit of very great deflection, 

 so that a great amount of work must be done upon it before 

 it slips from the pin, the primary current may have 

 attained its full strength before the slip takes place. 

 This corresponds to the capacity of the Leyden jar being 

 too great to allow of its being charged to a sufficient 

 potential to produce a spark. In this case no spark takes 

 place, but when the force between the wheels D and B 

 diminishes on account of the diminution of the acceleration 

 of P, the spring relieves its strain by forcing the wheel S 

 backwards, and the Leyden jar under corresponding circum- 

 stances quietly discharges itself through the secondary coil, 

 reversing the operation by which it was charged. But if the 

 pin slips from the spring, the wheel S will revolve, and the 

 spring will fly back corresponding to a disruptive discharge 

 through the air, and if the acceleration of P continue long 

 enough several such disruptive discharges may take place. 



We must, of course, be careful not to endeavour to learn 

 from such a model lessons which it was not designed to teach, 

 and we must remember that the behaviour of the mechanism 

 does not represent the electrical action in all respects. 



For many years Maxwell rendered valuable service to 

 the British Association, especially in connection with elec- 

 trical science. Some account of the meetings which he 

 attended will be found in the letters printed in another 

 part of this work, and though during the last few years 

 of his life other engagements prevented his attendance at 

 the annual gatherings, he always showed signs of keen en- 



