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But if a point p is connected to E, the sparking will cease; that is, the point will protect E from the discharges, even though the distance Cp be considerably greater than CE.

The lower end of a lightning rod should be buried deep enough so that it will always be surrounded by moist earth, since dry earth is a poor conductor. It will be seen, therefore, that lightning rods protect buildings not because they conduct the lightning to earth, but because they prevent the formation of powerful charges in the neighborhood of the buildings on which they are placed.


FIG. 234. Illustrating the action of a lightning rod

Flashes of lightning over a mile long have frequently been observed. Thunder is due to the violent expansion of heated air along the path of discharge. The roll of thunder is due to reflections from clouds, hills, etc.*



293. Potential difference. There is a very instructive analogy between the use of the word "potential" in electricity and "pressure" in hydrostatics. For example, if water will flow from tank A to tank B through the connecting pipe R (Fig. 235), we infer that the hydrostatic pressure at a must be greater than that at b, and we attribute the flow directly to this difference in pressure. In exactly the same way, if, when two bodies A and B (Fig. 236) are connected by a conducting wire r, a charge of + electricity

FIG. 235. Illustrating hydrostatic pressure

* A laboratory exercise on static electrical effects should follow the discussion of this section. See, for example, Experiment 27 of the authors' Manual.

is found to pass from A to B (that is, if electrons are found to pass from B to A) we say that the electrical potential is higher at A than at B, and we assign this difference of potential as the cause of the flow.* Thus, just as water tends to flow from points of higher hydrostatic pressure to points of lower hydrostatic pressure, so electricity tends to flow from points of higher electrical pressure, or potential, to points of lower electrical pressure, or potential.




FIG. 236. Illustrating electrical pressure

Again, if water is not continuously supplied to one of the tanks A or B of Fig. 235, we know that the pressures at a and b must soon become the same. Similarly, if no electricity is supplied to the bodies A and B of Fig. 236, their potentials very quickly become the same. In other words, all points on a system of connected conductors in which the electricity is in a stationary, or static, condition are at the same potential. This result follows at once from the fact of mobility of electric charges through conductors.

But if water is continuously poured into A and removed from B (Fig. 235), the pressure at a will remain permanently above the pressure at b, and a continuous flow of water will take place through R. So, if A (Fig. 236) is connected with an electrical machine and B to earth, a permanent potential difference will exist between A and B, and a continuous current of electricity will flow through r. Difference in potential is commonly denoted simply by the letters P.D. (Potential Difference).

*Franklin thought that it was the positive electricity which moved through a conductor, while he conceived the negative as inseparably associated with the atoms. Hence it became a universally recognized convention to regard electricity as moving through a conductor in the direction in which a + charge would have to move in order to produce the observed effect. It is not desirable to attempt to change this convention now, even though the electron theory has exactly inverted the rôles of the + and charges.

294. Some methods of measuring potentials. The simplest and most direct way of measuring the potential difference between two bodies is to connect one to the knob, the other to the conducting case,* of an electroscope. The amount of separation of the gold leaves is a measure of the P.D. between the bodies. The unit in which P.D. is usually expressed is called the volt. It will be accurately defined in § 334. It will be sufficient here to say that it is approximately equal to the electrical pressure between the ends of copper and zinc strips when dipped in dilute sulphuric acid

or to two thirds of the electrical pressure between the zinc and carbon terminals of the familiar dry cell.

Since the earth is, on the whole, a good conductor, its potential is everywhere the same (§ 293); hence it makes a convenient standard of reference in potential measurements. To find the potential of a body relative to that of the earth, we connect the outer case of the electroscope to the earth by means of a wire, and connect the body to the knob. If the electroscope is calibrated in volts, its reading gives the P.D. between the body and the earth. Such calibrated electroscopes are called electrostatic voltmeters. They are the simplest and in many respects the most satisfactory forms of voltmeters to be had. Their use, both in laboratories


FIG. 237. Electrostatic


*If the case is of glass, it should always be made conducting by pasting tin-foil strips on the inside of the jar opposite the leaves and extending these strips over the edge of the jar and down on the outside to the conducting support on which the electroscope rests. The object of this is to maintain the walls always at the potential of the earth.


and in electrical power plants, is rapidly increasing. They can be made to measure a P.D. as small as volt and as large as 200,000 volts. Fig. 237 shows one of the simpler forms. The outer case is of metal and is connected to earth at the point a. The body whose potential is sought is connected to the knob b. This is in metallic contact with the light aluminium vane c, which takes the place of the gold leaf.

A very convenient way of measuring a large P.D. without a voltmeter is to measure the length of the spark which will pass between the two bodies whose P.D. is sought. The P.D. is roughly proportional to spark length, each centimeter of spark length representing a P.D. of about 30,000 volts if the electrodes are large compared to their distance apart.


295. Condensers. Let a metal plate A be mounted on an insulating base and connected with an electroscope, as in Fig. 238. Let a second plate B be similarly mounted and connected to the earth by a conducting wire. Let A be charged and the deflection of the gold leaves noted.


If now we push B

FIG. 238. The principle of the condenser

toward A, we shall observe that, as it comes near, the leaves begin to fall together, showing that the potential of A is diminished by the presence of B, although the quantity of electricity on A has remained unchanged. If we convey additional - charges to A with the aid of a proof plane, we shall find that many times the original amount of electricity may now be put on A before the leaves return to their original divergence, that is, before the body regains its original potential.

We say, therefore, that the capacity of A for holding electricity has been very greatly increased by bringing near it another conductor which is connected to earth. It is evident from this statement that we measure the capacity of a body by the amount of electricity which must be put upon it to raise it to

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Great Italian physicist, professor at Como and at Pavia; inventor of the electroscope, the electrophorus, the condenser, and the voltaic pile (a form of galvanic cell); first measured the potential differences arising from the contact of dissimilar substances; ennobled by Napoleon for his scientific services; the volt, the practical unit of potential difference, is named in his honor

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