29O TRANS. ST. LOUIS ACAD. SCIENCE. 



a and c. Divide die 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 o 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. 



^. 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. 



5. It can instantly be converted into a differential 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 weighing 7.37 grams per meter, the resistance of which is 1 ohm to 

 192.9 cm. of wire (which is 96. 45 cm. on the instrument scale), and thousandths of an ohm 

 -•an be measured direct. 



