486 Mr. J. P. Joule on the Heat disengaged 



4. The galvanometer was, in all its essential parts, constructed 

 similarly to Pouillet's compass of tangents. A stout circular 

 mahogany board (aa, figs. 1 and 2, PI. XI.) supported upon three 

 leveling screws, forms the base of the instrument. To the diameter 

 of this a stout vertical board is fixed, having a semicircular hole 

 in its centre for the reception of the graduated circle bb. This 

 circle, constructed of brass entirely free from magnetic influence, 

 is 6 English inches in diameter and divided to half-degrees ; it 

 is also furnished with leveling-screws, &c. At ccc are three 

 screws furnished with nuts, for the purpose of affixing to the 

 vertical board any coil that may be required. 



The magnetic needle, of which I have given a full-size repre- 

 sentation in fig. 3, consists of two pieces of hard steel, each hah 

 an iuch long, kept about a millimetre asunder by the interven- 

 tion of a small piece of brass to which they are cemented. An 

 exceedingly delicate glass pointer*, weighing only 7 or 8 mil- 

 ligrammes, is affixed to the top of the needle. The ends of the 

 pointer are painted black, in order to be the more distinctly 

 seen as they traverse the silvered edge of the circle. A piece of 

 fine copper wire is affixed to the needle in such a manner as to 

 afford the means of suspension by a single filament of silk, and 

 also of a rest when the instrument is not in use. The filament 

 of silk is suspended from a graduated circle, dd, fig. 2, by means 

 of which torsion can be given to the filament. There is also a 

 piece of apparatus, ee, fig. 2, by means of which the needle can 

 be elevated, depressed, or adjusted to the centre of the divided 

 circle bb. 



One can, as I have already hinted, affix to the vertical board 

 of the galvanometer a coil consisting of a fewer or larger number 

 of turns of copper wire, according to the nature of the experi- 

 ment intended to be made. In the present research I employed 

 one consisting simply of a thick copper wire bent into a circle of 

 a foot (English) diameter. The connexion between the coil of 

 the galvanometer and the voltaic battery (which were placed at 

 a distance from each other of about six metres) was established 

 by means of clamps and screws at ff. 



I need hardly remark, that it was found necessary to protect 

 the galvanometer against any vibrations of the floor of the labo- 

 ratory in which the experiments were made. This was effected 

 by boring three holes in the floor, and driving strong wooden 

 stakes through them into the ground. The feet of the stool 

 upon which the galvanometer is placed rest upon the tops of 



* The use of a glass pointer was suggested to me by Mr. Dancer, the 

 maker of my galvanometer. I have since found that it has been employed 

 by Professor Buusen of Marburg in his galvanometer of tangents. 



