F. E. Nipher — New Jorm of Lantern Galvanometer. 113 



brass plugs, inserted at a. J, c, o?, e, ^, A, ^. Putting plugs at h 

 and e, and connecting the poles of a galvanic cup at the binding- 

 screws A and C, and the current runs successively through the 

 tv'O coils R, each causing deflection in the same direction. Let 

 R represent the resistance of one coil of the galvanometer, then 

 the resistance of the galvanometer will be 2R This arrange- 

 ment is used in working with ordinary galvanic currents. 



If instead of the former connections, plugs be put at a, c?, g, 

 and h, the wires from the source of electricity being connected 

 at E and F, then the galvanometer resistance becomes ^R. 

 This arrangement is to be used with circuits of small 

 such as therm o-currents. For this kind of work the i 

 is thoroughly adapted. 



This instrument can also be used as a differential galvanome- 

 ter. To do this, put the positive pole of the battery at E. 

 Plug 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 

 wire the resistance of which is to be determined, into one 

 branch, bring the needle to zero agam by introducing known 

 resistances into the other, and the unknown resistance is readily 

 determined. In measuring fractions of an ohm, a rheochord 

 is, all things considered, the best. The contacts are good, and 

 an audience obtains a better idea of what is meant by electrical 

 resistance than when a resistance box alone is used. Using 

 platinum wire weighing 7-37 grams per meter, the resistance of 

 which is one ohm to 192-9 cm. of wire (which is 9645 cm. on 

 tbe instrument scale), and thousandths of an ohm can be 

 measured direct. 



If ground connections are made the negative pole of the 

 battery is sent to ground direct, and the branches of the current 

 from B and D are sent to ground through the unknown resis- 

 tance and the resistance box respectively. 



Shunts may be introduced into either of the half circuits. 

 This may be done by introducing coils of resistance ^R or ^V-K, 

 between the binding screws A, B or C, D. These 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. Permanent shunts may be 

 introduced by connecting one extremity with plates A or D, the 

 other extremity 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 used in Latimer Clark's differential galvanom- 

 eter, and their use in measuring resistance is too well known to 

 need further explanation. 



The advantages possessed by this galvanometer are : 



Am. .Joiik. 8oi.— Thibt) Series, Vol. XI, No. 63.— Feb., 1876. 



