THE POTENTIOMETER. 2$ 



A troublesome problem was encountered in the division of the io-ohm 

 coils of box A into ioo parts to read to hundred thousandths of a volt. At 

 first it was thought that the double shunt device suggested by Richards and 

 Lewis * for the same purpose would be sufficient. 



The shunts were both standardized and a complicated method of applying 

 the corrections worked out. Needless to say, the labor of adjusting this 

 device was severe ; but unfortunately the results obtained with it were not 

 wholly consistent. For instance, the cups of the cell were filled with vari- 

 ous amalgams, which we will call A, B, C, and D. The potentials between 

 A and B, B and C, and C and D were separately determined, corrected, 

 and added up. Then the potential between A and D, derived in the same 

 manner, was compared with this sum. The resulting values were re- 

 spectively 0.05085, and 0.05081 nominal volt. Other similar experiments, 

 hardly more satisfactory than the one just described, led us to abandon the 

 method. Probably the fault lies in the excessive number of movable con- 

 nections necessary in the apparatus, which answers well when less accuracy 

 is demanded. 



The final arrangement is shown in figure 5. MN is a manganin wire of 

 nearly 10 ohms resistance and 90 cm. in length ; its extremities are soldered 

 to the upper surfaces of the brass plates at M and N. A long strip of glass 

 fits accurately between these, and forms a satisfactory bed for the wire. 

 The whole is mounted upon a well-seasoned slab of whitewood, bearing a 

 scale which divides MN into 100 equal parts. A heavy copper wire is sol- 

 dered at one end to M, and at the other to the heavy brass connector Y, 

 which can be clamped firmly upon any io-ohm peg in the box A. N was 

 connected to a resistance box, C, which contained 10, 20, 30, and 40 ohm coils. 

 The resistance from P to U, plus the small coil R, plus the slider, equals 

 nine times the resistance of box A. If, however, the movable connector Y 

 is moved from its present position in the course of a measurement, n times 

 10 ohms are cut out of the circuit. To maintain the rate of fall of potential 

 at its original value, the proper resistance is introduced from C. 



L was a Leclanche cell, whose potential was about 1.4 volts ; its current ran 

 through the parts of the system so far described, and also through a variable 

 resistance, E, of about 4,200 ohms. By properly adjusting E, the potential 

 between B and P could be made equal to that of the i-volt cell V within 1 

 part in 14,000. The switch S 1 is thrown toward K 1 , and when the i-volt cell 

 was just balanced by the fall of potential between B and P, no deflection of 

 the galvanometer G was noted on closing the key K r . The Helmholtz cell V 

 was made with great care from pure materials, as recommended by Ostwald ," 



Proc. Am. Acad., 34, 91 (1898). 

 Ostwald-Luther, Hand- und Hulfsbuch, p. 364. 



