VERIFICATION OF A RESISTANCE BOX. 373 



of the two wires are shown by the figure, the cups i and 2 serve as 

 axis for the commutator. 



According to the direction in which the rocker is inclined, the 

 cups i and 3 and 2 and 4, or i and 5 and 2 and 6 are connected. 

 In the first case the connections of the two circuits are AA' and 

 and MB'; in the second, MA' and BB', the latter of the two 

 combinations always containing the wire of the galvanometer. 



The rocker being in the first position, the contact B' is dis- 

 placed so that the current is very weak in the galvanometer. The 

 rocker is reversed, and the constant M is displaced until the play 

 of the rocker leaves the needle of the galvanometer stationary. 

 The resistances AM and MB are then equal. 



The connections are made by fine wires provided with small 

 weights at their free ends. These weights serve to stretch the 

 wires, which are simply placed at A and B. 



By means of the make and break keys K, K', and k, the currents 

 in the two principal circuits, and then in the galvanometer, are only 

 closed at the time of observation. 



The current may, moreover, pass continuously in the two cir- 

 cuits, the heating of the wire having no influence on the exactitude 

 of the method. The only condition is that the currents be constant. 



974. VERIFICATION OF A RESISTANCE Box. In accurate ex- 

 periments it is necessary to know the exact ratio of the coils in a 

 resistance box, and a well-constructed instrument should furnish 

 the means of making this verification. It is necessary by a suitable 

 system of contacts to be able to take the different coils separately. 

 We have seen that in Elliot's box (Fig. 181) the cavities in the plate 

 which form the top of each dial enable us by plugs furnished with 

 binding screws to take the resistance of any intermediate coil. 

 These coils may then be investigated separately or by the aid of 

 an auxiliary bridge. 



We first compare the different coils of the same set with each 

 other, one by one, two by two, etc., which furnishes a great num- 

 ber of equations of condition. A supplementary unit for instance, 

 that in the interior of the box at vv enables us then to form with 

 the first dial ten units, which are compared with the twelves. These 



P and N serve as axis for the rocker, which consists of two conducting arcs in- 

 sulated from each other, and communicating respectively with the cups P and N. 

 The terminals of the battery being connected at P and at N, the screw B 

 is positive for the position of the rocker shown in the figure, and negative if 

 the rocker is reversed so that the extremities of the arcs dip in the cups A' and B'. 

 For the preceding experiment one of the cross wires has been suppressed. 



