the Construction of Galvanometers, 4"c. 247 



guide such a needle is for determining tlie position of the galvanometer in order 

 to render it fit for use. Yet, as hitherto made, we have no other means of setting 

 the instrument in the magnetic meridian than to tiu'u it round on its axis until 

 the compound needle point north and south on the graduated circle. It is true 

 that so great au error will only be possible when the needle is brought unusu- 

 ally near the astatic state ; but it will be considerable in proportion to the sensi- 

 biUty attained. The magnetic meridian line and compass needle, which I have 

 added to the galvanometer, afford a protection against this source of fallacy. 



One immediate ill consequence of the error of the needle, should it exist 

 undiscovered and uncorrected, is, that it affects those degrees of the circle 

 which are of most value in delicate galvanometry. The first twenty degrees are 

 in the direct ratio of the deflecting force, which no other degrees on the circle 

 are. The effect of terrestrial magnetism is as the sine of the angle comprised 

 between the magnetic meridian and the magnetic axis of the needle on which 

 it acts. The sine of 20° is as nearly as possible double the sine of 10°; but 

 beyond 20" the virtual ratio cannot be determined without subniittincf each 

 particular galvanometer to an experimental investigation. A nd more than this, 

 if the experimenter shall have taken the trouble of ascertaining the value of 

 the degrees above the twentieth, by Melloni's, or any other method, it will 

 prove unavailing ; for the whole scale of ratios of deflecting forces to angular 

 deflections becomes deceptive if tlie magnetic meridian line of the galvanometer 

 do not correspond with the terrestrial magnetic meridian. When the instru- 

 ment is used as a galvanoscope, a small error is perhaps of little consequence, 

 except that it causes the needle to represent deflecting forces weaker or stronger 

 than they really are ; but when it is used as a measure of heat, and for some 

 other nice purposes, a want of coincidence between the magnetic meridian of 

 the instrument and the terrestrial magnetic meridian would be productive 

 of serious error. It is therefore evident how useful is the addition of the 

 magnetic meridian line, with its point and compass needle, for setting the gal- 

 vanometer due north and south : it at once detects the error of the astatic needle, 

 if such exist. 



On the subject of the coil, it maybe proper to mention that, althouo-h it is 

 often made of copper wire covered with cotton, such a covering is altogether 

 unfit. I have observed that one of my galvanometers, which is furnished 



