Electric Bells and All About Them

body in which the manifestations were first observed. The ancients were acquainted with a few detached facts, such as the attractive power acquired by amber after friction; the benu

rved the former of these facts, but nearly twenty centuries elapsed befo

r to, if not identical with, heat and light. These, like sound, are known to be dependent on undulatory motion; but, whilst sound is elicited by the vibration of a body

anical; 2nd, chemical; 3rd, changes of temperature. Among the mechanical may be ranged friction, percussion, vibration, trituration, cleavage, etc. Among the chemical we note the action of acids and alkalies upon metals. Every chemical action is accompanied by electrical effects; but

e methods of evoking electricity, so as to fam

pea, be suspended from the ceiling by a piece of fine cotton, previously damped and then approached by an ebonite comb which has been briskly rubbed, it will be vigorously attracted, and never repelled; but if for the cotton there be substituted a thread or fibre of very fine dry silk, the pith-ball will be first attracted and then repelled. This is owing to the fact that the damp cotton allows the electricity to escape along it: id est, damp cotton is a CONDUCTOR of electricity, while silk does not permit its dissipation; or, in o

ty to any practical purpose, the following table is subjoined, giving the names of the commoner bodies, beginning with those which most readily transm

CONDUCTORS A

Substance. Rela

tors Silver

anneal

hard dra

hard dra

anneal

ard dra

m, annea

presse

variabl

, anneal

n 6

, sof

lver alloy,

anneal

ressed

presse

ver (varia

ver alloy, 1

hard

, presse

ury

uth

ite 1

Acid 97

uctors Hydroch

acid 103

allic salts vari

c sulph

water [1]

tors. Metallic

ther forms of

tt

e

a

oho

er

Wood

Ic

ic Oxi

onduc

ors. Ice,

and o

uc 10000

cha 1000

ases, and

ol

130000

mon

lk

ss

x

phu

in

er

lla

n 15000

not been accu

as approximate only, since the conductivity of all these bodies varies very largely with their purity, and with the te

h which we are acquainted; while damp air, on the contrary, owing to the facility with which

been excited has also acquired electrical properties, attracting the pith-ball, previously repelled by the rod. From this we may gather that when one body acting on another, either mechanically or chemically, sets up an electrical condition in one of the two bodies, a similar electrical condition, but in the opposite sense, is produced in the other: in point of fact, that it is impossible to excite any one body without exciting a corresponding but opposite state in the other. (We may take, as a rough mechanical illustration of this, the effect which is produced on the pile of two pieces of plush or fur, on being drawn across one another in opposite directions. On examination we shall find that both the piles have been laid d

ce very considerably the result. For example: if two similar sheets of glass be rubbed over one another, no change in electrical condition is produced; but if one be roughed while the other is left polished, this latter becomes positively, while the former becomes negat

n in electric bell-ringing or signalling, we need not to go more deeply into this portion of the sub

ed, while the one less acted on becomes positive.[2] The following table, copied from Ganot, gives an idea of the electric

cid fluid. ? ↓ Zinc. ↑ ? The

admiu

Tin.

Lead

Iron

ickel

ismut

ntimo

opper

ilver

Gold

latin

raphi

f the list are positive to those below them, but negative to those above them, if the test have reference to the condition of the parts within the fluid. On the contrary, we s

g.

g.

ination (which constitutes a very primitive galvanic couple) be immersed in a tumbler three-parts filled with water, rendered just sour by the addition of a few drops of sulphuric or hydrochloric acid, we shall get a manifestation of electrical effects. If a delicately poised magnetic needle be allowed to take up its natural position of north and south, and then the wires proceeding from the two metal strips twisted in contact, so as to be parallel to and over the needle, as

tely deflected from this north and south position, and assumes a new direction, more or less east and west, according to the amplitude of the current and the nearness of the conductor to the needle. Moreover, the direction in which the north pole of the needle is impelled

entering current. If, on the contrary, the wire, or conductor, be placed below the needle, the deflection will, under similar circumstances, be in the opposite direction, viz.: the no

at § 9, it will be evident that inside the tumbler the zinc is positive to the copper st

re be placed in metallic contact with the wires leading from the simple battery described at § 9, and it will be found that the iron has become powerfully magnetic, capable of sustaining several ounces weight of iron and steel, so long as the wires from the battery are in contact with the wire encircling the iron; or, in other words, "the soft iron is a magnet, so long as an electric current flows round it." If contact between the battery wires and the coiled wires be broken, the iron loses all magnetic power, and the nail

retain its magnetism instead of losing it immediately on contact with the battery being broken; and, in the second place, the attractive power elicited would have been much less than in the case of soft iron. It is therefore of the

g.

howing Lin

Fig. 3 gives an idea of the distribution of the iron filings, and also of the general direction of the lines of force. It is found that if a body be moved before the poles of a magnet in such a direction as to cut the lines of force, electricity is excited in that body, and also around the magnet. The ordinary magneto-electric machines of the shops are illustrations of the application of this property of magnets. They consist essentially in a horse-shoe magnet, in front of which is caused to rotate, by means of appropriate gearing, or wheel and band, an iron bobbin, or pair of bobb

g.

, showing esse

e, two or more ends of which are brought out in a line with the spindle on which it rotates, and fastened down to as many insulated sections of brass cylinder placed around the circumference of the spindle. Two metallic springs, connected to binding screws which form the "terminals" of the machine, serve to collect the electrical wave set up by the rotation of the coiled cylinder (or "armature") before the poles of the electro-magnet. The annexed cut (Fig. 4) will assist the student in getting a clear idea of the essential portions in a dynamo:-E is the mass of wrought iron wound with insulated wire, and known as the field-magnet. N and S are cast-iron prolongations of the same, and

otate by connecting up the pulley at the back of the shaft (not shown in cut) with any source of power, a very small current is set up in the wires of the armature, due to the weak magnetism of the iron mass of the field-magnet. As this current (or a portion o

the armature; and these cumulative effects rapidly increase, until a limit is reached, dependent partly on the speed of rotation,

e out of the acid. For reasons which will be explained farther on, t

ar that what we usually term the negative is reall