30 RESEARCHES ON II. 



variation ; now the time occupied in a series of experiments was scarcely an hour, 

 after sufficient skill was acquired. 



The element of Daniell, used for the experiments on the globe, was composed of 

 a copper cylinder eight inches (20 centimet.) high, and six inches wide. With a 

 hollow cylinder of amalgamated zinc, equally high, and three inches in diameter 

 (7.61 centimet.), the deviation was fixed to 20° of a galvanometer, bearing a rather 

 short wire, equal in resistance to five turns of the rheostat, all the resistance of 

 the circuit being seventy turns. In one experiment, the circuit was kept close 

 during two days, after which the deviation had diminished 2°, and six turns more 

 were to be uncoiled from the rheostat, to reduce the needle to the former deviation. 

 After fourteen days, the current went on, not very much weakened, when, no more 

 sulphate having been added, the almost pure water remained. Then a curious 

 anomaly took place, viz., that the resistance of the pair appeared variable, and 

 seemed to increase when the interposed resistance was greater. Whatever may be 

 the cause of this irregularity, which deserves to be studied, it is evident that we 

 can rely on the constancy of the battery for the time of the experiments. 



To be more certain of this, a galvanometer was constantly kept for this purpose 

 in the circuit, which was carefully observed at the beginning and at the end of 

 every series. Besides, no series was commenced without keeping the circuit 

 closed for a certain time, until the battery reached its constant state. If the 

 difference between the observation of the strength of the battery made at the 

 beginning and at the end of any series was greater than two turns of the rheostat, 

 that series was excluded as not being a good one. 



II. The galvanometers interposed in the circuit were Nobili's multipliers, some 

 of which were constructed under his own direction, but their sensibility was too 

 great for these experiments; this was diminished either by taking away the 

 second needle, or disposing it at right angles with the other, or wholly inverting it. 

 I preferred galvanometers with short and light needles, because they become sta- 

 tionary very soon, when those with long and heavy needles require a longer time. 

 Now to save time in this matter is highly important, in order to avoid errors 

 arising from a change of strength in the battery. Also, for the purpose of 

 checking the vibrations of the needle, the graduation of the circles was made on 

 pure copper plates. In the system of experiments which we have adopted, there 

 is no need of observing every time on the galvanometer the absolute deviation of 

 the needle, but only to see whether the current has the same degree of force ; if not, 

 it is to be reduced to the same intensity by regulating the length of the connecting 

 wire. Hence the observations are free from errors due to imperfect graduation, or 

 the eccentricity of the needle. To avoid also errors of parallax, the needle was 

 observed through a microscope with crossed wires. (See PI. III., Fig. II.) The 

 magnifying power of the instrument was also very useful in appreciating better the 

 least variations of the needle. As the diameter of the circles was greater than the 

 length of the needles, a very fine glass wire was fastened to it with a little wax, in 

 order to have a very nice index. By all these means together, a variation of a 

 needle not greater than the finest line in a graduated circle could be distinguished 

 very readily. 



