18 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



f ig> 43a Galvanometer. (Fig. 



433.) Greater mobility 

 may be given to the 

 needle by the more de- 

 licate mode of suspen- 

 sion employed in the 

 balance of torsion. With 

 this view, it may be sus- 

 pended at its centre by 

 a fine thread, or, what 

 is best of all, by a single 

 filament of silk, enclosed 

 in a tube, and attached 

 to the lower end of a 

 short metallic wire, pass- 

 ed through the cover 

 which crosses the top of 

 the tube, and capable 

 of being turned in the 

 aperture with some de- 

 gree of friction, so as to bring the needle to any required 

 horizontal position. The angular turning requisite for this 

 purpose is marked by an index fixed upon the upper end 

 of the wire, by reference to a small graduated circle imme- 

 diately below it, in the upper side of the cover. All these 

 parts are represented in the vertical section. The other 

 parts of the apparatus, as far as relates to the coils of wire 

 which encircle the needle, are similar to those of the former 

 instrument : excepting that the wires in the middle of the 

 upper part of the coil are separated a little, in order to leave 

 an opening for the free passage of the thread that supports 

 the needle. A graduated circle, equal in diameter to the 

 length of the needle, is placed immediately below the needle. 

 The compass, wire, and card, are en- 

 closed in glass in order to secure 

 them from the agitations of the air, 

 and to allow of our seeing the posi- 

 tion of the needle. 



Price, $8.00. 



Galvanometer, with Astatic Needle. 

 This figure (Fig. 434) represents a 

 simple arrangement of the galva- 



Fig. 434. 



