ELECTRO-MAGNETIC INSTRUMENTS. 43 



mented together in that form ; used with the ribbon coil 

 the experiments are very interesting and astonishing. 



The experiments of Professor Henry, relative to the induc- 

 tion of secondary currents at a distance, are exceedingly 

 striking. By sending an intermitting current of electricity 

 through the spiral, a, and placing the helix of thin wire, b, 

 over it, a plate of glass being interposed, shocks may be ob- 

 tained on grasping the handles attached to the coil. When 

 a consisted of about 300 feet of copper ribbon, one and a 

 half inch in width, and b, a helix of copper wire five miles 

 long, Dr. Henry found that shocks might be obtained when 

 the coils were four feet apart ; and at a distance of twelve 

 inches, they were too strong to be taken through the body. 

 The Professor also mentions a very instructive method of 

 exhibiting these astonishing experiments, which the author 

 has frequently adopted in the lecture room, viz. to cause 

 the induction to take place through the partition-walls of 

 two rooms, for which purpose a coil, about 100 feet of 

 ribbon, is suspended against the wall in one room, while a 

 person in the adjoining room receives the shock by grasping 

 the handles of a helix, of about 300 yards of thin wire, and 

 approaching it to the spot opposite to which the coil is sus- 

 pended. The effect is as if by magic, without a visible 

 cause. It is best produced through a door, or thin wooden 

 partition. Price, $3.00 to $6.00. 



