10. If a boy who had magnetized his knife blade in a physics laboratory, pointed end south-seeking, should lose his way in the woods on a cloudy day, how could he determine his way out? 11. At a certain point the earth's field acts north, that of an electric current, east. The magnetoscope needle points exactly northeast when placed at that point. How do the two magnetic fields compare? (2) ELECTRICAL MEASUREMENTS =263. Galvanometers.=--In using electric currents it is often necessary or desirable to be able to know not only that a given current is weak or strong, but precisely what its strength is. We can determine the relative strengths of two currents by the use of a _galvanometer_. [Illustration: FIG. 243.--The magnet is at the center of the coil.] [Illustration: FIG. 244.--A moving-magnet (tangent) galvanometer.] The older or _moving-magnet_ type of galvanometer is similar to the galvanoscope mentioned in Art. 239. It consists of a magnetic needle mounted at the center of a coil of wire. The coil is placed facing east and west, so that the needle will be held by the earth's magnetic field parallel to the plane of the coil. When a current is sent through the coil a magnetic field is produced within it. This deflects the needle, its north end turning east or west depending upon the direction of the current. (See Fig. 243.) The _coils_ of a moving-magnet or _tangent_ galvanometer (see Fig. 244) are _large_ and firmly fastened to the base, while the _magnet_ is _small_. The _moving-coil_ type of galvanometer (see Fig. 245) consists of a _large magnet_ fastened to the frame of the device. The magnet usually has a horseshoe form to produce as strong a field as possible. The _coil_ is wound on a _light_ rectangular frame and is suspended between the two poles of the magnet. To concentrate the magnetic field, a cylinder of soft iron is usually placed within the coil. Fig. 246 represents a common form of moving-coil galvanometer. [Illustration: FIG. 245.--To illustrate the principle of the moving-coil galvanometer.] [Illustration: FIG. 246.--A moving-coil (D'Arsonval) galvanometer.] =264. Measurement of Electric Currents.=--A galvanometer enables one to _compare_ electric currents. To _measure_ electric currents it is necessary to employ a _unit_ of electrical quantity, just as in measuring the quantity of water delivered by a pipe, a unit of liquid measure is employed; thus, _e.g._, the current delivered by a given pipe may be 2 gallons of water per second, so in measuring the flow of an electric current one may speak of two _coulombs_ per second. The _coulomb_ is the unit quantity of electricity just as the unit of quantity of water is the gallon. For most practical purposes, however, we are more interested in the _rate_ or _intensity_ of flow of current than in the actual _quantity_ delivered. The unit of rate of flow or current is called the _ampere_.