138 Dr. J. A. Fleming on the Use ofDanieWs Cell 



without much affecting its temperature, and the potential 

 measured at the extremities of this resistance by means of a 

 comparison on the potentiometer between it and that of the 

 Daniell, this gives at once the current. 



In order to construct a resistance whose value at the in- 

 stant of passing the current can be accurately measured, the 

 following device is adopted : — On a board are mounted a 

 series of copper blocks a, a, 6, 6, like the connection-pieces on a 

 resistance-box (see PL V. fig. 3). These can be connected by 

 plugs />, p. At opposite sides and ends are main terminals as 

 M 7 ; and the blocks a, b are connected by brass or German- 

 silver wires wound in a spiral; similar coils of equal resistance 

 connect each pair of opposite blocks. The number of coils 

 must be an odd number. 



If the plugs 1, 11, 3, 9, 5, 7 are put in, it is obvious all the 



coils Rj, R 2 . . . R 7 are a series, and their resistance can be 



measured : call it r. If, then, the other pegs are inserted, the 



coils are now in multiple arc or parallel, and if there are n 



v 

 coils the resistance is — 2 . By this arrangement the resistance 



in parallel can be inferred from the resistance in series ; and 

 although each coil has a very small resistance, yet by using a 

 large number, not only is a very easily measurable resistance 

 obtained when the coils are in series, but a resistance of large 

 current-carrying capacity when they are put in parallel. 



The mode of using it is as follows : — The pegs are all put 

 in, and the current to be measured is sent through the coils in 

 parallel. By means of the potentiometer and standard-cell, 

 the difference of potential is found between the terminals M 

 and W when the current is passing. The alternate pegs are 

 then removed, the current stopped, and the resistance taken 

 of the coils in series, and the time noted which has elapsed 

 between the measurement of potential and of the resistance. 

 A few observations are taken of the resistance at intervals of 

 time, by which to construct a curve of cooling; and by projecting 

 back the curve, it is easy to ascertain very nearly the resistance 

 at the time when the potential was measured. If small currents 

 only are employed, it is not necessary to change the resistance; 

 the current can be sent through it in series, and its resistance 

 also measured in series. The writer has had constructed a 

 large resistance of this kind at the Victoria Electric Lighting 

 Station, for the measurement of dynamo currents up to 500 

 amperes. In this case the resistance consists of 36 wires of 

 brass, No. 14 B.W.G., arranged in parallel, and of equal 

 resistance. One of the wires can be disconnected and arranged 

 instantly on a bridge to measure its resistance ; and in this 



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