[Illustration: Fig. 611.--Diagram showing connections of Leeds and Northrup potentiometer. The coils in the series AD are each 5 ohms, and between each two there is a brass block with a reamed hole. A pair of flexible cords with taper plug terminals to fit these holes is furnished. These coils can be measured with an ordinary Wheatstone bridge and thus compared with each other to a high degree of accuracy, even if the bridge be not accurate. For potentiometer work, the essential point is that they should be like each other, not that they should be accurately any particular value. In the same way the resistance of the extended wire can be compared with the resistance of the coils in AD. Its resistance should be 1.1 times the value of any coil between A and D. Outside connection with the extended wire may be made by using the posts marked BR and -BA. This adjustment for balancing an unknown electromotive force is accomplished by the manipulation of the two contact points M and M'. The coils AD are arranged in a circle, a revolving switch moving M. A checking device enables the operator to set this switch without taking his eye from the galvanometer. The resistance S is of such value that when it shunts the wire OB, the total resistance between O and B is 1/10 of the same unshunted. When the shunt is applied, provided the total current remain the same, the drop between any two points on AB will be 1/10 of its previous value. The total current will remain the same provided the total resistance in the circuit remain the same. This is accomplished by making the coil K such that it exactly compensates for the reduction in resistance caused by plugging in the shunt coil S. The low scale is applied by moving the plug from the position 1 to the position .1. With this change the potentiometer reads from .16 volt down by indicated steps of .000005 volt. The reading is very simple. For instance, if M stand at 1.2 and M' at 1.35 revolutions, the reading is 1.2135 volts. The resistances of the instrument are wound upon metal spools, and are therefore able to dissipate a comparatively large amount of energy. This allows the potentiometer to be used for pressure measurements up to 16 volts without the use of a volt box.]