Comment about the Fiske-Mott insulator

The Fiske-Mott Insulator.

In your issue of December 1st, you take a certain position with reference to the Fiske-Mott corrugated insulator, which seems to me hardly tenable.

You say, "Theoretically speaking, two wires, one laid upon and at right angles to the other should not be in electrical contact, since the contact takes place at a mathematical point, but actually the contact may be of practically zero resistance. The same is the case with insulators, for actually the surfaces in contact though apparently very small, are quite sufficient to enable the current to pass freely, and thence to spread all over the surface of the porcelain, and thus get to earth."

In the first place, two wires placed as you indicate, would not have contact in a mathematical point only, because wires not being mathematically perfect cylinders, their sections are not mathematically perfect circles; wires not being, moreover, of absolute hardness, there must be a slight compression of the material at the place of contact.

But it is impossible that the resistance of the contact should be practically zero. Nearly every telephone transmitter at present in use, depends for its action upon the fact that the resistance of contact of two electrodes is not practically zero, but that it is an absolute quantity, which varies in amount as the area and pressure of contact are varied.

You are correct in saying " the surfaces in contact (in an insulator) although apparently very small, are quite sufficient to enable the current to pass freely and thence to spread all over the surface of the porcelain and thence get to earth."

Whenever any path is offered to a current, a current can pass. But to say that the degree of freedom with which the current can pass is not influenced by the area and pressure of contact, would be to utter a very startling statement.

The leakage of ten insulators on a telegraph line is about ten times that of one insulator, because the area of contact is about ten times as great. In the Fiske-Mott insulator, the area of contact of each insulator is divided—say by a; therefore the resistance of a of those insulators is about the same as that of one ordinary insulator.

The insulator is not, however, made of porcelain, as you seem to suppose ; porcelain allows a film of moisture to collect on its surface, and is therefore undesirable. The material used by the Chicago Insulating Company is designed with especial reference to preventing the forming of such a film of moisture.

Bradley A. Fiske.

Washington, D.C.,

Dec. 12th, 1883.

[We will reply to Mr. Fiske's letter in our next issue.—EDS. ELEC. REV.]