CITAP. I.] ELECTRICITY AND MAGNETISM. 35 



were darting off from every part of its outer surface. 

 Into this cube Faraday took his most delicate electro- 

 scopes ; but once within he failed to detect the least 

 influence upon them. 



32. Applications. Advantage is taken of this in 

 the construction of delicate electrometers and other 

 instruments, which can be effectually screened from 

 the influence of electrified bodies by enclosing them 

 in a thin metal cover, closed all round, except where 

 apertures must be made for purposes of observation. It 

 has also been proposed by the late Prof. Clerk Maxwell 

 to protect buildings from lightning by covering them 

 on the exterior with a network of wires. 



33. Apparent Exceptions. There are two ap- 

 parent exceptions to the law that electricity resides only 

 on the outside of conductors, (i) If there are electrified 

 insulated bodies actually placed inside the hollow con- 

 ductor, the presence of these electrified bodies acts in- 

 ductively and attracts the opposite kind of electricity to 

 the inner side of the hollow conductor. (2) When 

 electricity flows in a current, it flows through the sub- 

 stance of the conductor. The law is limited therefore 

 to electricity at rest, that is, to statical charges. 



34. Faraday's "Ice-pail" Experiment. One ex- 

 periment of Faraday deserves notice, as showing the 

 part played by induction in these phenomena. He 

 gradually lowered a charged metallic ball into a hollow 

 conductor connected by a wire to a gold-leaf electro- 

 scope (Fig. 21), and watched the effect. A pewter ice- 

 pail being convenient for his purpose, this experiment is 

 continually referred to by this name, though any other 

 hollow conductor a tin canister or a silver mug, placed 

 on a glass support would of course answer equally 

 well. The following effects are observed : Suppose 

 the ball to have a + charge : as it is lowered into the 

 hollow conductor the gold leaves begin to diverge, for 

 the presence of the charge acts inductively, and attracts 



