Contact, Electric.
A contact between two conductors, such that a current can flow through
it. It may be brought about by simple touch or impact between the ends
or terminals of a circuit, sometimes called a dotting contact, or by a
sliding or rubbing of one terminal on another, or by a wheel rolling on
a surface, the wheel and surface representing the two terminals.
There are various descriptions of contact, whose names are
self-explanatory. The term is applied to telegraph line faults also, and
under this, includes different descriptions of contact with neighboring
lines, or with the earth.
Contact Electricity.
When two dissimilar substances are touched they assume different
electric potentials. If conductors, their entire surfaces are affected;
if dielectrics, only the surfaces which touch each other. (See Contact
Theory.)
Contact Faults.
A class of faults often called contacts, due to contact of the conductor
of a circuit with another conductor. A full or metallic contact is where
practically perfect contact is established; a partial contact and
intermittent contact are self-explanatory.
Contact Point.
A point, pin or stud, often of platinum, arranged to come in contact
with a contact spring, q. v., or another contact point or surface, under
any determined conditions.
Contact Potential Difference.
The potential difference established by the contact of two dissimilar
substances according to the contact theory, q. v.
Contact Series.
An arrangement or tabulation of substances in pairs, each intermediate
substance appearing in two pairs, as the last member of the first, and
first member of the succeeding pair, with the statement of the potential
difference due to their contact, the positively electrified substance
coming first. The following table of some contact potentials is due to
Ayrton and Perry:
CONTACT SERIES.
Difference of Potential in Volts.
Zinc--Lead .210
Lead--Tin .069
Tin--Iron .313
Iron--Copper .146
Copper--Platinum .238
Platinum-Carbon .113
The sum of these differences is 1.089, which is the contact potential
between zinc and carbon.
Volta's Law refers to this and states that--
The difference of potential produced by the contact of any two
substances is equal to the sum of the differences of potentials
between the intervening substances in the contact series.
It is to be remarked that the law should no longer be restricted to or
stated only for metals.