Chemical Science. 171 



shall induce the whole arrangement to return to a constant posi- 

 tion when uninfluenced by electrical currents, and yet combining 

 with the lower needle, to cause deflexion when an electrical cur- 

 rent is passing through the wire. 



As an illustration of the delicacy of the instrument M. Nobili 

 observes, that it is well known if Seebeck's combination of anti- 

 mony and bismuth be attached to a common galvanometer, and 

 the point of junction be cooled, only a very slight effect is ob- 

 served on the instrument ; whilst, if attached to the new galva- 

 nometer, the same influence is sufficient to make the needles re- 

 volve several times. If a piece of iron wire, five or six inches long, 

 be used to connect the extremities of the copper wire of the in- 

 strument, by twisting the ends together, and one of the points 

 of contact be warmed by touching it with the hand, the needle 

 will move from 0°, and in the first oscillation extend to 90°. 

 Even the mere approximation of the hand to the junction of the 

 metals will produce a deviation of 20°. 



It is necessary for the delicacy of the instrument that the 

 needles used be magnetized as nearly as possible to the same 

 degree, and two indications have been observed as useful in point- 

 ing out when this is the case ; the first is the position taken up 

 by the plane of the needles, when left to the earth's influence ; 

 this should not be in the plane of the magnetic meridian, but 

 more or less inclined to it ; the second is the manner in which 

 the system oscillates about its line of equilibrium. These oscil- 

 lations should be very slow compared with those of a common 

 needle. 



In consequence of the situation of the graduated circle above, 

 and not within the frame, the folds of the wire may be brought 

 much nearer to each other than in the common instrument ; this 

 renders it more compact, and from the vicinity of the needle 

 within to the wire, also more powerful. When fixing the gradua- 

 tion, the zero should be placed so as to accord with the position of 

 the needles, when left to the earth's influence ; this will not be 

 towards the true magnetic north, but will not be far from it, 

 and will always be constant. 



M. Nobili then offers a very curious illustration of the powers 

 of the instrument : — " It is known," he says, ** that water 

 usually retains itself at a lower temperature than the ambient 

 air, the difference being sometimes two degrees, and resulting 

 from the evaporation of the liquid. If a bar of bismuth be 

 made to join the two extremities of the galvanometer wire, and 

 one of the points of junction be plunged into a cup of water, 

 the needle will immediately deviate several degrees, proving that 

 the instrument is capable of measuring the small degree of re- 

 frigeration, produced by the evaporation of the liquid. I have 

 actually submitted one of my galvanometers for fifteen days to 



