XVIII INTRODUCTION. 



THE UNIFILAR MAGNETOMETER. 



DESCRIPTION OF THE INSTRUMENT. 



The unifilar magnetometer consists of a 12-inch azimuth circle-, divided on silver, and read 

 by three verniers to 10". It has a tangent screw and clamp. Above the circle a brass plate is 

 centred which carries an oblong rectangular box of gun-metal with apertures at the extremities, 

 fitted with plates of parallel glass. The box is surmounted by a glass suspension tube nine 

 inches long, with a sliding suspension pin and torsion circle. Its sides are of wood, removable 

 at pleasure. The torsion circle is divided to four degrees, and may be read by a vernier to one 

 degree. A binding screw holds the sliding pin in place when at a proper elevation, and is pre- 

 vented from turning .within its socket by a projecting screw-head fitting into a groove cut its 

 entire length. A fixed level is attached to the plate, and there are levelling screws beneath 

 the circle. 



There are four projecting arms to the plate. One pair sustains a reading telescope, with a 

 divided ivory scale permanently placed in the vertical plane of its focus, and its counterpoise ; 

 and the second pair at right angles to the former support a gun-metal rod carefully divided 

 into feet and decimals of a foot. The rod is about three and a half feet in length, and is held 

 in place, edgewise beneath the box, by thrumb-screws that pass through the arms-. A frame 

 support for the deflecting magnet slides upon the rod and serves to determine its distance with 

 accuracy. 



The magnets are hollow cylinders, each resting in a stirrup fitted with a small mirror below 

 its centre, for reflecting the divisions of the scale above the eye-end of the telescope. The sus- 

 pension magnets are three inches, and the deflecting magnets are three inches and sixty-seven 

 hundred ths in length. Their stirrups and mirrors are removable, and the former are so con- 

 structed that they may be attached to the suspension fibre without torsion. 



A brass cylinder of (nearly) the same dimensions and weight as the suspension magnet ac- 

 companies the instrument, to be used in determining the plane of detorsion; and it has a ther- 

 mometer for registering temperature ; a portable tripod stand; a small lamp for observations at 

 night or in a dark room ; a brass plummet ; a reel of unspun silk ; extra magnets of both di- 

 mensions ; suspension tubes and adjusting tools. 



THE VIBRATION APPARATUS. 



The vibration apparatus consists of a rectangular mahogany box upon a table-top, which is 

 centred on a board of equal dimensions. The table-top moves freely in azimuth _, but the two 

 may be firmly clamped together and the lower one made horizontal by its levelling screws. 

 There are glazed apertures in the ends of the box, movable sides for ready manipulation of the 

 magnet, and a glass tube for the suspending fibre screws into the top. The tube is furnished 

 with a torsion circle cap and a sliding suspension pin, in all respects similar to that of the 

 unifilar magnetometer. 



During experiments of vibration the 3. 67-inch magnet rests in a stirrup carrying a mirror, 

 and its oscillations are noted by a telescope and scale, which is similarly adjusted and of like 

 dimensions to those of the unifilar. The telescope is attached to an arm of the table-top, and 

 is counterpoised by a movable weight, and its scale serves also to measure the arc of vibration. 



