Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . g-netic needle when it is at rest,so that the plane of the coil passesthrough the axis of the needle andthe magnetic meridian. Galvanom-eters are usually set up to conform to the above conditions beforesending a current through them; the currents field will thenact at right angles to the earths field, and the position theneedle will take up when the current flows is the resultant ofthese two forces. A certain amount of the currents field istherefore used toovercome the earthsattracti
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . g-netic needle when it is at rest,so that the plane of the coil passesthrough the axis of the needle andthe magnetic meridian. Galvanom-eters are usually set up to conform to the above conditions beforesending a current through them; the currents field will thenact at right angles to the earths field, and the position theneedle will take up when the current flows is the resultant ofthese two forces. A certain amount of the currents field istherefore used toovercome the earthsattraction for theneedle before itmoves at all. Whenthe needle is deflect-ed to 90 degrees,or at right anglesto the wire, it is inthe maximumposition of the cur-rents field. Thevalue of the deflec-tion is dependentupon the currentflowing through thecoil, but is not pro-portional to the current; that is, if one current produces twicethe number of deflections of another current the former is notof twice the strength, ^[ 182. With the needle parallel tothe coil, or at the zero scale position, a small current deflects. s^ .Oj~. ^ c. - • /Ste S&t&S, ue*W&«f»<r< Fig. 154. -Construction of Students DetectorGalvanometer. GAL VANOMETERS. 169 it considerably, but as the angle the needle makes with thecoil increases, a much greater magnetic force is example, it requires a greater force to deflect the needleone degree from the 45-degree position than to deflect it onedegree from the 15-degree position. The galvanometer coilmay be wound with a great many turns of fine wire, in whichcase the instrument is said to be sensitive ; that is, the needleis appreciably deflected by a very small current; or it may becomposed of a few turns of very heavy wire, in which case itis intended for use with large currents. By the sensibility of a galvanometer is meant the amount ofcurrent required to produce a given deflection. The sensi-bility is sometimes rated in ohms. For example, a galvan
Size: 2225px × 1123px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublishernewyo, bookyear1901
