126 ANNUAL OF SCIENTIFIC DISCOVERY. 



lique direction, conformably to the result of the combined actions of 

 the current and the earth. The more the action of the current pre- 

 dominates, the nearer this result approaches the perpendicular of the 

 conducting wire; so that the greatness of the deviation produced by 

 the influence of the current, serves to measure the current itself, pro- 

 vided the observer knows the law which determines the intensity, and 

 its relation to the greatness of the deviation two quantities which 

 vary in the same sense, but which are never rigorously in proportion. 

 When the needle and the current are placed in certain conditions 

 (which will be presently specified,) the angular deviations increase 

 less rapidly than the intensity of the current ; but if instead of the 

 angles of deviation, their trigonometrical tangents are taken, the ex- 

 perimenter will find values in proportion to the intensity he seeks to 

 ascertain. To do this, the electrical current must move in a vertical 

 circle, whose plane is parallel to the natural direction of the magnetic 

 needle ; in other words, the circular conductor must be placed accor- 

 ding to the magnetic meridian ; secondly, the magnetic needle placed 

 in the centre of this circle, must be so small as not to move beyond 

 the plane of the circle (during its oscillations from right to left,) 

 where the maximum of the magnetic actions take place. Then, 

 and only then, M. Pouillet's tangent compass gives, without need 

 of correction, exact measures. 



But, it is evident that a galvanometric compass, formed of a cir- 

 cle infinitely large, or of a needle infinitely small, is a pure fiction ; 

 and so soon as an experimenter wishes to realize it, and gives it 

 such proportions as are necessary for convenience in experiment- 

 ing, the circle is always too small or the magnetic needle too 

 large. Then between the real and the theoretical deviation such 

 great differences are found to exist [differences which augment 

 in proportion to the strength of the currents] the experimenter is 

 forced to choose, as recently happened with M. Despretz, between 

 two equally inconvenient alternatives : to restrict the use of the com- 

 pass to the measurement of small arcs of deviation, or to calculate 

 greater deviations by a complicated formula, in which new terms are 

 employed, charged with representing the characteristic data of the in- 

 strument. Besides, it will easily be conceived that all sensible in- 

 crease of size given to the needle placed in the center of the circular 

 current, militates against the exactness of the deviations ; in truth, 

 the principle of the compass-tangent supposes that in every position 

 of the magnetic needle around its pivot, the sum of the distances of 

 each of its points to all the elements of the circle remains the same. 

 But it is clear that the moment the needle quits the plane of the cir- 

 cle in turning on its pivot, its extremities are removed from the ma- 

 jority of the points, which must then influence it at its then distance ; 

 and, consequently, this influence diminishing the deviation, must re- 

 main less than its theoretical value. Thus all tangent-compasses 

 hitherto employed, are the more inert, as they are used to measure 

 stronger currents. 



Such was the state of the question when M. Gaugain, an engineer 



