A NKW CURRENT WEIGHER, ETC. 



409 



the ampere to a very high degree of accuracy. In fact, this unit is now known with 

 a precision considerably greater than any other electrical quantity of which absolute 

 measurements have been made. 



SECTION 2. GENERAL DESCRIPTION. 



The instrument consists of a very sensitive physical balance supporting a coil with 

 vertical axis from each end of the beam, these coils hanging coaxially within fixed 

 coils carried from the base of the balance. A diagrammatic sketch of the arrangement 

 is shown in fig. 1, and a view of the complete instrument in fig. 2, Plate 7. 



From the former it will be seen that the current flows in opposite directions in 

 the upper and lower parts of the outer coils. On the left-hand side of fig. 1 the 

 current in the upper half of the outer coil flows clockwise (looking from above) and in 

 the lower half counter-clockwise, whilst in the left-hand suspended coil the circulation 

 is shown clockwise. The tendency is, therefore, to lift the suspended coil SL. It 



Fig. 1. Diagram of windings. 



Fig. 3. Diagram showing hollow 

 cylinder with double winding 

 in grooves of screw threads. 



will also be seen that the outer coils on the right will tend to depress the suspended 

 coil SR, so that the two sets of coils exert a clockwise torque on the beam. This 

 torque is balanced by weights added to or taken from scale pans supported 

 independently on the knife edges which carry the suspended coils, an arrangement 

 which avoids displacement of the suspended coils when the weights are placed or 

 removed. 



All the coils are wound with bare wire on hollow marble cylinders, having double- 

 threaded screw grooves cut on the surfaces, into which separate wires are laid as 

 shown in fig. 3. In this figure one wire is indicated by two thin lines, and the other 

 is shown thick. The two wires, hereafter distinguished as No. 1 and No. 2, form two 

 adjacent helices, which, in the use of the instrument, are connected in series and act 

 as one coil. They can, however, be readily disconnected from each other and an 



