the Construction of Galvanometers, 6fe. 241 



the centre than those on the upper circle, to enable the observer to take the 

 two corresponding degrees in his view, along with the interposed points of the 

 needle, when his eye is directed vertically over the four objects, so that they 

 shall all coincide. As a further facility, the numbers are engraved inside the 

 circle. The instrument stands on levelling screws, which are received in holes 

 in a brass ring screwed to a wooden stand ; and the whole is covered by a 

 French shade. A stop which acts underneath the stand checks tedious oscil- 

 lations. 



From one of the three studs which support the upper circle, proceed 

 horizontally two long brass blades hinged to it in such a manner that they can 

 be made to approach towards or recede from each other. When brought toge- 

 ther, they fit closely by their straight edges, and constitute a kind of Ibrceps 

 capable, by its long handles, of firmly grasping the spindle of the compound 

 needle when it is suspended in the centre of the graduated circles. By means 

 of a sliding clip, they can be retained in this position. The silk fibre, which 

 sustains the compound needle, is suspended from a pin in a cross bar at the top 

 of a pillar, in the same manner as in the galvanometer, and is similarly circum- 

 stanced in all respects with regard to the adjustments which have been already 

 mentioned. A compound needle, with its silk fibre and pin, may thus be trans- 

 ferred from one instrument to the other, and will fit exactly in either. 



Experiments made with the volta-magnetometer soon convinced me that the 

 apparently anomalous position assumed by the needle, which I had taken so 

 much pains to render astatic, and which nevertheless stood perpendicularly to 

 the magnetic meridian, was in strict accordance with the circumstances under 

 which it was placed. To explain the matter, it is necessary to advert to what 

 the properties of a compound needle would be, if, as its name expresses, it were 

 really astatic. If the four poles of the needle, supposed to be perfectly similar 

 in magnetic power, be placed in the same vertical plane, with the synonymous 

 poles contiguous to each other, the directive power will be at its maximum. If 

 in this state of things the bars be opened out, no matter what the angles sub- 

 tended by them may be, the law of the parallelogram of forces comes into ope- 

 ration: the compound needle will take such a direction, that the resultants of 

 the parallelograms, two sides of which are the intersecting magnetic axes, will bi- 

 sect the vertical angles which include the magnetic meridian, and will, therefore, 



