TO THE MOVEMENT OK MACHINES. 3 



four inches in length, balanced on a very fine pivot, at the 

 centre of a circle divided into semi-degrees, is placed in a 

 box, furnished Avith three adjusting-screws, so as to secure a ho- 

 rizontal position. Instead of the usual multiplying coil, which 

 would too much weaken the cuirent, there is only a single brass 

 wire, of 1^ line in thickness, which passes exactly through the 

 centre below the needle. The extremities of this wii-e are fur- 

 nished with two small cups containing a few drops of mercury, 

 and which allows the circuit to be easily completed. The bottom 

 of the box consists of a glass plate, not a metallic one, as it is 

 usually made ; a precaution which I have thought necessary for 

 magneto electric experiments. This galvanometer is neverthe- 

 less so sensitive as to be affected by very weak electric forces. 

 Although the deviations of the needle cannot serve as an exact 

 measure of the force of the current, they however indicate the 

 slightest variation the battery has undergone during the progress 

 of the experiments. By employing a single voltaic pair, of small 

 dimensions, as Professor Parrot has done, there are many means 

 of restoring, at any time, the original state, and of having a con- 

 stant force. Lastly, with respect to the importance of these ex- 

 periments, it would be no great loss to reject a paii- of plates which 

 have become worn, and to substitute fresh ones in their place. 

 Moreover, we may convince ourselves that it is far more neces- 

 sary to keep account of the temperature of the conducting liquid 

 than to clean the pair after each experiment, &c. But whatever 

 pains we may take to operate under similar circumstances, we 

 must recollect that the experiments cannot but be incorrect, un- 

 less we employ the galvanometer. 



The most sure and exact method for these experiments, is to 

 try the magnetic power of two or three bars at a time. The 

 helices are united into a single conjunctive wdre so as to submit 

 these bars constantly to the influence of the same current. By 

 interposing a galvanometer, which is indispensable, any varia- 

 tion in the action of the battery may even be turned to account ; 

 for by this means, we learn what relation exists between the re- 

 lative power of the different portions of the circuit and the in- 

 tensity of the current. Trifling differences in the construction 

 of the helices may be removed by some method of elimination 

 or by reciprocal combinations. The details into which 1 have 

 entered will, I trust, be excused, as it is often impossible to 



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