182 DOVE ON THE ELECTRICITY OF INDUCTION. 



tion of p—A is not here obtained, because, during the rotation 

 of the keeper through the nearest semicircle, the voltameter, as 

 in § 84, is directly interposed in the circuit of the current, as 

 the keeper also remains in the connexion. This is not the case 

 when the voltameter is inserted between II and III instead of 

 being placed between I and III. Here the spiral is in the con- 

 nexion, and the keeper remains excluded. If the commutation 

 here happens a little too late, we therefore obtain no action, and 

 with I and III, according as the previous current is active or 

 not, a variable or invariable quantity of gas when iron is in- 

 serted into the spiral. A further prosecution of this investiga- 

 tion therefore did not appear advisable, as the slightest alteration 

 in the point of contact of the spring upon the cyhnder exerts 

 a considerable influence. 



4. Galvanometer. 



87. As with continuous sliding springs alternating currents 

 succeed each other, we obtain in this case, even when the gal- 

 vanometer* effects a secondary connexion, the phaenomena of 

 the so-called deviation in a twofold direction, according to which 

 the needle is moved in the same direction to that which it al- 

 ready occupied towards the coils of the galvanometer, before 

 being acted upon by any current -n hatever. Something similar 

 naturally happens, when with a spring that breaks connexion 

 twice (in azimuth 90° and 270°), the wire of the galvanometer, 

 during the rotation of the keeper through the second and fourth 

 quadrant, does not form a secondary but the chief connexion. 

 In this latter case therefore the phaenomena are more pro- 

 minent. 



88. If, on the contrary, the intermittent spring (3) breaks con- 

 nexion only once in azimuth 90°, then in like manner the gal- 

 vanometer is traversed by alternating currents, but only when 



* In' galvanometric experiments the extra spiral must be considerably re- 

 moved from the Saxton's machine. For as soon as the rotating keeper is 

 brought from its horizontal position before tlie poles of the magnet into a verti- 

 cal position, the magnetism in the magnet becoming free exerts an inducing 

 influence upon the iron which is contained in the extra spiral. 'J'he current 

 thence induced in the spiral is perceptible at considerable distances when the 

 galvanometer needle is truly astatic. To ascertain the distance at which this 

 disturbing influence ceases, we have onlj'to connect the ends of the extra spiral 

 in the first instance alone with the galvanometer, and then to turn the keeper. 

 If no action tlien ensues, the spiral must be at the proper distance from the 

 machine. 



