particles coming in contact with each other and the earth generate electricity, which accumulates in clouds and is then surrendered to objects having a lower potential, such as other clouds and the earth.

Dry snow driven by the wind creates electric excitement and gives eredence to this theory.

State the use of lightning-rods.

They are good electric conductors which tend to guide the course of lightning from the atmosphere to the earth and thus protect objects to which they are attached. Electricity always follows the path of least resistance, and, therefore, will follow a good conductor in preference to a bad one. The rods are usually made of steel or copper held in place by insulated supports. The lower end should penetrate moist earth or some other good conductor, while the upper end should terminate in a point projecting several feet above the adjacent parts of buildings. What space is protected by a lightning-rod?

The space protected is a field, nearly conical, whose height and basal radius are the height of the rod.

What is the duration of an electric spark?

It is very brief, lasting from 23 to 46 ten-millionths of a second, the time being prolonged by the distance through which the spark travels. This is ascertained by rapidly revolving a card attached to clock-work, each of whose sectors is painted a color in the order and shade or tint of the spectrum. The blending produces a gray color until illuminated by an electric spark. The colors then stand sharply defined as if in a state of rest, showing that the spark has come and gone before the card had time to move.

What is the aurora borealis ?

Owing to the fact that the magnetic needle and other electric apparatus are disturbed by its presence the luminous effects seen in the northern region are believed to be of electric origin.

How may a body be protected from electric influence?

It is shown that the electroscope, enclosed by a metal case or a screen of copper gauze, is unaffected by external electricity. Excited bodies presented to the gauze, discharge themselves through the metal without influencing the electroscope. This principle has its application in protecting storages of explosive and inflammable material.

DYNAMIC, VOLTAIC, OR CURRENT ELECTRICITY.

How does static electricity differ from Voltaic?

It differs in its origin and constancy. The former is a condition of tension, and its escape is instantaneous or nearly so. The latter is generated by some action tending to produce a current, which flows without cessation until the generators are withdrawn or enfeebled.

Describe a Voltaic cell.

C

FIG. 39.

By placing a strip of copper and one of zinc (Fig. 39) in a vessel containing dilute sulphuric acid, and connecting them above the liquid by a metal conductor, a current or flow of electricity is produced. Volta discovered this action by using an even number of alternating plates of zinc and copper separated by cloth wet with the acid, the first and last plates being connected by wire. The acid acts more vigorously upon the zinc than upon the copper, and creates within the liquid a current of electricity toward the copper and through the wire a current toward the zinc. This action continues while the plates are connected and the material unexhausted. Zinc in a pure state is quickly consumed by the action of the acid; hence to avoid its rapid dissolution it is usually dipped in mercury previous to using. The current engendered is feeble compared with the discharge of a Holtz machine.

Of what metals may a battery be constructed?

W

Simple cell.

As

In order to establish a current, it is necessary that one of the two metals selected have a greater affinity than the other for the acid. a matter of economy, preference should be given to the durability of the metals. If chemical action create equal potentiality in the two metals, there will be no current.

Explain the difference between quantity and intensity.

The normal charge which any body possesses at all times and under all circumstances is its quantity of electricity. This charge may be neutral or excited. In the latter case, either the + or - is in excess; and if one species were entirely withdrawn and the other substituted in its place, the quantity would remain invariable, but the kind, and therefore the intensity, tension, or density, would be increased. Intensity is the available excess of one of the electric fluids.

How are quantity and intensity obtained?

By increasing the size of the plates used an increase of quantity is obtained. Tension or intensity, however, depends on the number of cells employed, five hundred producing an electric arc several inches in length, while fifty make a very serviceable battery.

What is polarization of the plates?

The chemism of zinc and sulphuric acid is to produce zinc sulphate and liberate hydrogen gas. The latter collecting in bubbles on the surface of the copper plate, decreases or totally destroys its efficiency by offering resistance or causing a counter-current, a condition termed polarization of the poles.

How is the difficulty of polarization obviated?

It is diverted by introducing into the battery a porous cup and some

oxidizing substance to unite with the
liberated hydrogen, and thus prevent
the accumulation. Many kinds of
cells have been constructed on this
principle. We therefore have
fluid" and "two fluid" batteries.
Describe Daniell's battery.

66

one

It is the standard of batteries, and one of great constancy. It consists of amalgamated zinc, Z, and copper, C (Fig. 40). A glass cell contains saturated cupric sulphate, S. Inside of the copper cylinder fits a porous cell containing sulphuric acid, A, and into this the zinc Z dips. The free hydrogen passes from the zinc to the cupric sulphate, producing sulphuric acid and freeing copper, which implate. Its electro-motive force or dif

Describe the Leclanche battery.

Liquid, saturated solution of ammonium chloride. Electro-positive metal, amalgamated zinc. Electro-negative element, a porous cell occupied by a carbon plate, the remainder being filled with oxide of manganese, in coarse powder, and small pieces of gas-carbon. Electro-motive force, 1.32.

Describe Grove's battery.

Dilute sulphuric acid is placed in the glass cup S (Fig. 41). In this is put the zinc Z, also the porous earthenware jar N, containing nitric acid and platinum, P. Electro-motive force, 1.82.

FIG. 41.

Z

What is the Grenet or bichromate battery?

It is a battery much used by physicians, and consists of two plates, one of zinc and one of carbon, contained in a liquid whose proportions are one pound of sulphuric acid, one pound of potassium bichromate, and 20 pounds of water. Describe the arrangement of cells in a battery.

Cells may be so adjusted as to augment either the intensity or the quantity of electricity. If quantity be desired from any number of cells,

say 40, all the zincs would be united for one pole and the carbons for the opposite. The intensity would be that of a single cell, but the quantity equal to 40. Now if required to obtain the greatest intensity, the cells would be coupled alternately. In a Grove battery it would be zinc, sulphuric acid, nitric acid, platinum; zinc, sulphuric acid, nitric acid, platinum, and so on for 40 variations. In this case the theoretical value of the result would be: quantity 1, intensity 40.

Again, a battery might be coupled in two series of 20 each, giving quantity 2, intensity 4; or four, giving quantity 4, intensity 10; or the opposite, viz., quantity 10, intensity 4.

It is understood that full values are never obtained in practice, resistance and other causes reducing them materially.

When the positive plates are connected by one wire and the negative plates by another, they are joined in multiple arc.

They are joined in a series when the positive plate of one cell is connected to the negative plate of the next.

What is the resistance of a cell?

There are two kinds of resistance, external and internal. The former is that which impedes the current tending to flow outside of the cell or away from it; the latter is that resistance encountered within the liquid and plates constituting the cell. Their difference is the difference of work done within the cell or battery compared with that done outside. The external resistance compared with a standard of resistance can easily be measured.

What is the unit of resistance or electric measure?

The unit of resistance is called the ohm. It is the resistance offered by a column of pure mercury 104.81 centimetres long, 1 square millimetre in section, at a temperature of 0° Centigrade. For practical use, coils of wire having this same resistance are used. The wire is an alloy of two parts of silver to one of platinum. As the resistance differs with the temperature of the wire, each coil is marked with the exact temperature at which the resistance is one ohm.

Boxes containing coils of varying lengths, and therefore of different resistances, are also constructed. They may measure fractions or multiples of an ohm. This unit is called the B. A. or British Association unit. It was established in 1860.

At what rate does resistance increase and decrease?

(1) Calling the electro-motive force that which tends to impel a flow of electricity from point to point, its constancy induces a constant current in a given wire.

(2) Doubling the length of the wire doubles the resistance, while it halves the current.

(3) The electric resistance is increased by diminishing the circumference of the conducting wire, and vice versa. The conditions of length,

18-C. & P.

material, etc., remaining the same, halving the cross-section doubles the resistance and halves the current.

(4) All resistances may be measured by the ohm. They depend upon the material relative to its length, cross-section, and conductivity.

How do we measure electro-motive force?

(1) The standard chosen for the unit of electro-motive force or power to overcome resistance is called the volt. It is a little less than the force of a Daniell cell, which is from 0.98–1.08_volts.

(2) The weber is the unit of current. It is the electro-motive force of a volt working through the resistance of an ohm.

The current is directly proportional to the electro-motive force and inversely proportional to the resistance. Any two of these terms being known determines the third.

What is the formula for resistance and conductivity?

The resistance is the reciprocal of the conductivity; or

Internal resistance: =

KP.

How is internal resistance measured?

Distance between the plates

Area of the plates under the liquid)

P is a number taken from a table based on the diameter, length, and material of wire according to known laws.

What are divided circuits or currents?

If the electrodes or polar wires be branched or split in several parts, the current will also divide, flowing through each branch in inverse proportion to their resistance. With equal divisions of the wire each section will receive the same fraction of the entire current, and in all cases the currents will be inversely as the cross-section of the branches. This knowledge is often applied to measure currents which are very strong. It is also serviceable to divide a current whose parts are to do work independently.

What is the galvanometer?

It is a mechanism to determine the strength and direction of a current (Figs. 42 and 43). A wire placed above or below a magnetic needle and