Polytechnic Association. 



951 



to twice its present force, which will be 3, and will be represented by 

 the line A C, the combined forces of AM and A C will direct the 

 needle toward the point 2. If we now lay a protractor on the circle, 

 we find that the line A 2 cuts it at about 63° 30', of which the tan- 

 gent is 2. 



We may increase the parallelogram eiected upon A M at pleasure, 

 and the two forces combined will always so balance the needle between 

 them as to make it point from A diagonally across the parallelogram 

 to its opposite angle, the height of which is the tangent of the angle 

 of deflection. 



By inspection of the diagram it is seen that the law holds good in 

 the subdivisions of the force as at .5, .25 and .125 — a truth admitted 

 by all experimenters as to the relations up to 14°. 



13. The rheostat contains coils whose several resistances range 

 from -j-^- of an ohm to 4,000 ohms, any one or more of which may 

 be thrown into the circuit by removing the proper plug or plugs on 

 the top of the rheostat, so that any resistance may be introduced 

 from -j-J-^ of an ohm to 10,000 ohms. 



14. In addition to two screw-cups (I and II) for connection with 

 the battery and galvanometer, there are two other screw-cups (III and 

 IY) for connection of any conductor whose resistance it is intended 

 to measure ; also a switch (A) so arranged that the battery current 

 may be directed at pleasure through the rheostat or the conductor. 

 The whole apparatus is packed in a morocco case, nine inches in 

 diameter and seven inches deep, having a handled strap, rendering it 

 convenient for transportation from place to place. 



