71. Current Strength. The strength of a current at the end of a circuit depends not only upon the electrical pressure, or E. M. F., which drives the current, but also upon the resistance which has to be overcome. The greater the resistance the weaker the current at the end of its journey.
72. Unit of Current Strength; The Ampere. A current having an E. M. F. of one volt, pushing its way through a resistance of one ohm, would have a unit of strength, called one ampere. This current, one ampere strong, would deposit, under proper conditions, .0003277 gramme of copper in one second from a solution of copper sulphate.
73. Measurement of Current Strength. A magnetic needle is deflected when a current passes around it, as in instruments like the galvanometer. The galvanoscope merely indicates the presence of a current. Galvanometers measure the strength of a current, and they are made in many forms, depending upon the nature and strength of the currents to be measured. Galvanometers are standardized, or calibrated, by special measurements, or by comparison with some standard instrument, so that when the deflection is a certain number of degrees, the current passing through it is known to be of a certain strength.
Fig. 67.
Fig. 67 shows an astatic galvanometer. Fig. 68 shows a tangent galvanometer, in which the strength of the current is proportional to the tangent of the angle of deflection. Fig. 69 shows a D'Arsonval galvanometer, in which a coil of wire is suspended between the poles of a permanent horseshoe magnet. The lines of force are concentrated by the iron core of the coil. The two thin suspending wires convey the current to the coil. A ray of light is reflected from the small mirror and acts as a pointer as in other forms of reflecting galvanometers.
Fig. 68.
74. The Ammeter, Fig. 70, is a form of galvanometer in which the strength of a current, in amperes, can be read. In these the strength of current is proportional to the angular deflections. The coils are made with a small resistance, so that the current will not be greatly reduced in strength in passing through them.
