=Note.=--The standard resistance will have to be arranged to suit each particular case to make the calculations even approximately correct. (See Exp. 129, Note.) The standard resistance may be increased by adding the various coils and rheostat wires, their values being known. _=349. Summary of Laws of Resistance.=_ 1. _The resistance of a wire is directly proportional to its length, provided its cross-section, material, etc., are uniform._ =EXAMPLE.= If 39.1 ft. of No. 24 copper wire has a resistance of 1 ohm, 78.2 ft. will have a resistance of 2 ohms, because 78.2 is twice 39.1; 70.38 ft. will have a resistance of 1.8 ohms, as (70.38 ÷ 39.1 = 1.8) it is 1.8 times 39.1. 2. _The resistance of a wire is inversely proportional to its area of cross-section._ The areas of cross-section of round wires vary as the squares of their diameters; so _the resistance of a wire is also inversely proportional to the square of its diameter, other things being equal_. =EXAMPLE.= A No. 30 wire has a diameter of about .01 inch, while the diameter of a No. 24 wire is about .02 in.; that is, the No. 24 has _twice_ the diam. that the No. 30 has. The area of cross-section of the No. 24, however, is four times that of the No. 30, so its resistance is but 1/4 that of the No. 30, the lengths, etc., being the same. (See Wire Tables.) 3. _The resistance of a wire depends upon its material, as well as upon its length, size, etc._ 4. _The resistance of a wire depends upon its temperature._ (See Elementary Electrical Examples.) CHAPTER XX. CURRENT STRENGTH. _=350. Strength of Current.=_ The water in a certain tank may be under great pressure, but if it is obliged to pass through long tubes before it can turn a water-wheel, for example, it is evident that the work done will depend not only upon the pressure in the tank, but upon the resistance to be overcome before the water gets to the wheel. The work that the water can do depends upon its _rate of flow_, and may be used to measure the _strength_ of the current. The strength of a current of electricity is measured also by the _work_ that it can do, and it depends upon its _rate of flow_ at the point measured. The strength may be determined from its magnetic, heating, or chemical effects. _=351. Unit of Current Strength; The Ampere.=_ A current having the strength of 1 ampere, when passed through a solution of silver nitrate under proper conditions, will deposit 0.001118 gramme of silver in _one second_; if passed through a solution of copper sulphate, copper plates being used for the electrodes, in the solution, 0.0003277 gramme of copper will be deposited in _one second_. (See Chemical Effects of the Current.) The thousandth of an ampere is called the milliampere. The strength of a current is proportional to the amount of chemical work that it can do per second. (See § 357.)