MAGNETIC INSTRUMENTS. 7 



Horizontal steel bar needles, prepared for delicate investi- 

 gations. These needles are constructed of various shapes 

 and dimensions, mounted with central agate, or ruby caps, 

 and every precaution taken in selecting the finest quality of 

 steel for their construction, and its treatment afterwards, in 

 the formation of the needles, also in the method employed 

 in the magnetization. Price, $2.50 to $5.50. 



Brass stand, with fine steel point, for the suspension of 

 horizontal magnetic needles. 



Price, $1.00 ; wood stand with steel point, $0.25. 



Fig. 414. 



Fig. 415. 



The Astatic Needle. (Fig. 414.) Two needles, perfectly 

 similar ii| their form, and magnetic intensity, are turned 

 with their poles in opposite directions, and placed upon the 

 same axis perpendicular to their length. 



The adjustment of the opposing polarities of the two 

 needles should be such, that the directive power of the 

 combination resulting from the magnetism of the earth is 

 very nearly balanced ; the compound needle being allowed 

 to retain only sufficient power to bring it to a constant posi- 

 tion when uninfluenced by any electrical current. This 

 needle is very sensitive to even a feeble galvanic current, and 

 is used in delicate galvanometers. 



Price, $2.00 to $5.00. 



Dipping Needle. (Fig. 415, as above.) This magnetic 

 needle, the cap of which is mounted in small gimbals, or 

 a universal joint, admits of the needle moving in a vritical 



