ELECTRO-MAGNETIC INSTRUMENTS. 



Fig. 427. 



15 



The battery required to perform this experiment with 

 advantage should be of a power not less than three of 

 " Grove's series," of the usual size. 



Price, $1.75, $3.00, and $5.00. 



Helix on Stand. (Fig. 428, as above.) The helix con- 

 sists of a coil of insulated copper wire, bound in the form of 

 a close spiral spring, in one or several layers, so as to form 

 a hollow cylinder, into which an iron rod or any body to be 

 magnetized^ is to be placed. Those having a number of 

 layers, and the internal diameter small, have great mag- 

 netizing power. Price, $2.50 and $3.00. 



Circular Magnetic Coil (Fig. 429, next page) Consists 

 of a flat coil of insulated copper wire, rolled up, for the 

 purpose of transmitting a current of the electric fluid. One 

 end of the coil is pointed, and mounted on a metallic centre, 



