mid on a Battery with invariable Currents. 359 



facility, although this is an object which we should keep con-, 

 stantly in view while studying the action of the electric forces. 

 I have endeavoured to compare the electro-magnetic effects 

 of a current by means of weight. The following is a descrip- 

 tion of the apparatus I employ. I take an assaying balance so 

 delicate that it will be turned by a fraction of a milligramme^. 

 At each end of the beam there is suspended from a vertical 

 pin a scale pan with a magnet having its north pole hanging 

 downwards. I then fix upon an apparatus properly con- 

 structed, two glass tubes of such a bore that the two magnetic 

 bars may be passed into them without touching their concave 

 surfaces. Around each of these tubes there is wound a cop- 

 per wire covered with silk, and so long as to form ten thousand 

 circumvolutions. After having placed the bars in the direction 

 of the axis of the spirals, 1 cause an electric current to pass 

 through the wire. Let us, at first, observe a single spiral ; 

 it is evident that the magnetized bar, as well as the beam with 

 which it communicates, will rise or fall according to the di- 

 rection of the current. Let us now, so place the second spiral 

 that the beam wmU move in the same direction, when the wire 

 is traversed by the current, and bring the two spirals to com- 

 municate with each other ; the actions of both upon the bars 

 will then necessarily be excited. The use of the apparatus 

 will be best illustrated by a few examples. Having taken two 

 plates (one of zinc and the other of copper), each presenting 

 a surface of 4? square centimetres (=1*6 inch nearly), and 

 being in communication with the two spirals, I immersed them 

 simultaneously into ten grammes of distilled water: one of the 

 scales rose, and it was found necessary to place in it a weight 

 of 2™*5 (= '0385 grain) in order to restore the equilibrium. 

 The magnetic needle of a short- wired multiplier, which had 

 been placed in the circuit, made a deviation of 60 degrees. 

 A drop of sulphuric acid being added to the liquid, it was 

 found necessary to employ 35™-5 (= '546 grain) in order to 

 preserve the equilibrium, l^he two currents were therefore 

 nearly in the ratio of I to 14. 



I subsequently endeavoured to ascertain in weight the ratio 

 of currents issuing from batteries composed of elements more 

 or less numerous. With a pile of 40 elements charged with 

 water, containing ^^^ sulphuric acid, ^V sea salt, and some drops 

 of nitric acid, 615 milligrammes (= 9*471 grains) were re- 

 quired to preserve the equilibrium. Hence it follows that this 

 current is to the current obtained by means of a single pair in 

 the ratio of IV^^ to \. 



For the purpose of measuring the thermo-electric currents, 



♦ '0154 grain. 



