Measure of the relative Tension of Electric Currents. 271 



inch wide and 2f long. When in use, this vane of tinfoil 

 works in a glass cup, k k, 3^ inches in diameter, which is filled 

 with water. 



One of the chief improvements in the instrument is con- 

 nected with the needle, and the axis on which it works. The 

 latter is a small cylinder of ivory ; it has two flat faces filed 

 upon it, corresponding to the direction of the needle. On each 

 of these faces, as is represented in fig. 3, is drawn a vertical 

 line, and a little to the right of it are placed five dots. The 

 polar extremities of the needle are accommodated with two up- 

 right wires p'p'i pp'^ an inch long, which serve as indexes; 

 and at a distance of 10 or 15 inches, in the magnetic meri- 

 dian, a plate of metal, not shown in the figure, with a small 

 hole in its centre, is placed to be used as a sight. When an 

 observation is to be made, the experimenter adjusts this sight 

 in front of the instrument, either on its 'north or south side ; 

 and on looking through it, as soon as the needle moves, he 

 sees the index pp^ traverse before the scale on the axis. There 

 is no shake or vibration, even though any one should cross 

 the floor or jar the table, for the index and the scale equally 

 participating in all these disturbances, the motion is almost 

 as steady as that of a shadow on a sun-dial ; the vane of tin- 

 foil does not in the least interfere with the accuracy of indica- 

 tion, but effectually stops the oscillations, and the utmost ac- 

 curacy may be obtained, by previously giving the index /?j9' 

 a slight bend out of the vertical line, and using the five dots 

 as a diagonal vernier. 



In the following memoir it will be seen, that the terms 

 primary and secondary wire are occasionally used ; the 

 former in a somewhat extended sense. I mean by it not only 

 the thick polar wires that come from the electromotor, those 

 which were used being one-fifth of an inch thick, but include the 

 electromotor itself, no matter what its character may be — if a 

 hydro-arrangement, the plates, exciting liquid, &c. The se- 

 condary wires are simply long or slender wires to obstruct 

 the current; of these I have occasionally used two, the first 

 47 inches long, the second 290 : they are of copper, one foot 

 of which weighs 10"65 grs., and are covered with silk. 



And lastly, the measures are sometimes arranged in a form 



such as this, 100 ) .ennn 



50/ ^°^^ 

 in which the large or upper number represents the quantity 

 passing the primary wire, the under or smaller number the 

 quantity passing the secondary wire; and the decimal on the 

 right hand of the bracket, being the quotient of the former 

 numbers, is, as will presently be shown, the representative 

 of the tension. 



