Page:Elementary Text-book of Physics (Anthony, 1897).djvu/371

§ 302] will be a flow of heat across the strip and the temperatures of its edges will differ. This difference of temperature is due to a cooling of one edge of the strip. These effects are not reversible in the sense in which the Peltier effect and the thermoelectric effect are reversible, but J. J. Thomson has shown that they are consistent with each other.

302. Measurement of Current.— Instruments which are used to detect the presence of a current, or to measure its strength, by means of the deflection of a magnetic needle, are commonly called galvanometers.

The simplest form of the galvanometer is the instrument called the Schweigger's multiplier. It consists of a flat spool upon which an insulated wire is wound a number of times. The plane of the coils is vertical, and usually also coincides with the plane of the magnetic meridian. A magnetic needle is suspended in the interior of the spool. When a current is passed through the wire, the needle is deflected from the magnetic meridian. Usually, in order to make the indications of the apparatus more sensitive, a combination of two needles is used. They are joined rigidly together, so that when suspended the lower one hangs in the interior of the spool, and the other in the same plane directly above the spool. These needles are magnetized so that the positive end of one is above the negative end of the other. If they are of nearly equal strength, such a combination will have very little directive tendency in the earth's magnetic field. It is therefore called an astatic system. When a current passes in the wire, however, the lines of force due to the current form closed curves passing through the coil, and both needles tend to turn in the same direction. Since the earth's field offers almost no resistance to this tendency, an astatic system will indicate the presence of very feeble currents. The apparatus here described is no longer used to measure currents, but only to detect their presence and direction.

The tangent galvanometer is that form of galvanometer which is commonly used to measure electrical currents in electromagnetic units. We will consider it only in one of its simplest forms. In