290 TRANS. ST. LOUIS ACAD. SCIENCE. 



a and c. Divide the negative wire into two equal branches, 

 which are to be connected at B and D. The circuit being thus 

 closed, the needle evidently remains at zero. Introducing any 

 w re, the resistance of which is to be determined into one branch, 

 the needle is of course deflected. Bring the needle to zero again 

 by introducing known resistances into the other branch, and we 

 at once determine the unknown resistance.* Shunts may be intro- 

 duced into either of the half .circuits. This may be done by intro- 

 ducing coils of a resistance \ R, or -^^ R, between the binding- 

 screws A, B or C, D. The wires may also be wound upon metallic 

 plugs which have been split lengthwise, the parts being insulated, 

 and each being connected with one extremity of the wire. Perma- 

 nent shunts may be introduced by connecting one extremity with 

 plates A or D, the other being attached to an insulated plate to 

 be put in contact with B or C by means of a solid metallic plug. 

 These shunts are the same as those used in Latimer Clark's dif- 

 ferential galvanometer, and the manner of using them in deter- 

 mining resistances is too well known to need further explanation. 

 The advantages possessed by this instrument are — 



1. It is easily adjusted to any vertical lantern, from which it 

 can be removed in a moment if desired. 



2. The distance of the deflecting coils from the needle being 

 readily varied, it can be adjusted to currents of various intensity. 



3. The resistance of the galvanometer is quickly varied from 

 one-half to twice the resistance of one of the galvanometer coils. 



4. The coils may be replaced by others when desired. 



jj. It can instantly be convei'ted into a difterential galvanometer, 

 and used in measuring electrical resistances. 



6. It can be constructed in any work-shop, at a very small 

 expense. 



St. Louis, Oct. 25, 1875. 



* In determining fractions of an ohm, it is convenient to use a rheocord, made of plati- 

 num wire. Using wire weigliing 7.37 grams per meter, the resistance of which is i ohm to 

 192.9 cm. of wire (which is 96.45 cm. on the instrument scale), and thousandths of an ohm 

 can be measured direct. 



