All substances do not allow the electric current to pass through them with the same ease. Even the liquid in a cell tends to hold the current back and offers _internal resistance_. The various wires and instruments connected to a cell offer _external resistance_. (See "Study," Chapter XVIII., for experiments, etc.) =69. Unit of Resistance.= =The Ohm= is the name given to the unit of resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in. of No. 24 copper wire, will make a fairly accurate ohm. _Resistance coils_, having carefully measured resistances, are made for standards. (See "Apparatus Book," Chapter XVII., for home-made resistance coils.) Fig. 64 shows a commercial form of a standard resistance coil. The coil is inclosed in a case and has large wires leading from its ends for connections. Fig. 65 gives an idea of the way in which coils are wound and used with plugs to build up _resistance boxes_, Fig. 66. =70. Laws of Resistance.= 1. The resistance of a wire is directly proportional to its length, provided its cross-section, material, etc., are uniform. 2. The resistance of a wire is inversely proportional to its area of cross-section; or, in other words, inversely proportional to the square of its diameter, other things being equal. [Illustration: Fig. 65.] 3. The resistance of a wire depends upon its material, as well as upon its length, size, etc. 4. The resistance of a wire increases as its temperature rises. (See "Study," Chapters XVIII. and XIX., for experiments on resistance, its measurement, etc.) [Illustration: Fig. 66.] =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. [Illustration: Fig. 67.]