30 Mr. Ritchie on a Torsion Galvanometer. 



astatic. Portions of a brass tube, about an inch long, are to be 

 soldered to the ends of the wires forming the rectangle, for the 

 purpose of holding a small quantity of mercury, to render the 

 metallic contact complete. The annexed cuts exhibit a vertical 

 section of the rectangle, and a horizontal one of its upper side. 



II 



The wires, forming the rectangle, are pressed close together, 

 and secured by a waxed sewing-thread, rolled tightly round 

 them. The rectangle is then fixed in a rectangular box, having 

 the upper side formed of two sliding panes of window glass, 

 for the purpose of shutting up the needle from the agitation of 

 the air. Each pane has a small semicircle cut out of the middle 

 of the edge, by means of a round file, so as to leave a circular 

 opening directly above that in the rectangle. Various contri- 

 vances for suspending the magnetic needle might be adopted. 

 The following is perhaps the most convenient : Into a strong 

 wooden sole, or base, fix two upright supports about three feet 

 long. A small stage at the top, having a divided circle on 

 its upper side, and which may be elevated or depressed at 

 pleasure, completes the frame of the instrument. The stage 

 has two holes of the same size as the supports, and at the 

 same distance, with two small screws passing through its sides, 

 opposite the centres of the openings, for the purpose of fixing 

 the stage securely at the proper height. A small cylindrical 

 wooden key or peg, having a small bore in the axis for the 

 purpose of receiving the end of the glass thread, passes through 

 the centre of the divided circle, and is made to turn easily, 

 without much friction. 



After numerous trials, the following appears to me the best 

 mode of preparing the threads of glass, so as to have their 

 extremities somewhat thick and tapering, for the purpose of 

 securing them in the torsion key, and in the axis which carries 



