N. R. Woodward is the author of The Glass Insulator in America, 1988 Report and developed the Consolidated Design Numbers identification system for glass insulators. 

Questions for this month's column were submitted by Dan French, Madison Heights, Virginia. Kindly send your questions to Box 1003, St. Charles, IL 60174

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During what time periods did Brookfield and Hemingray each offer the CD 145 insulator to their customers?

The earliest CD 145 insulators are those made by Brookfield with the 45 Cliff Street address and the January 25, 1870 patent date. These have the very narrow wire groove, incorporating the British theory of that period; that of having the groove diameter very close to the size of the heavy iron wire used on telegraph lines. 

Since the office was moved to 45 Cliff Street in 1882, we may assume that these first insulators were made at about that time. The 1870 patent date, issued to Homer Brooke, would have indicated use of the press equipment and method of manufacture that it covers; thence the "swirl start" threads.

The November 13, 1883 and February 12, 1884 dates which appear on insulators made during the middle and late 1880's were issued to Samuel Oakman, and refer to the double petticoat feature and to the precise design, respectively. During those years there was fierce competition, and court battles were fought over rights to various features covered by the patents. It would seem that Brookfield may have actually been making a double petticoat insulator prior to the Oakman patent: but that patent also covers another feature which would conceivably allow its consideration as being "new" even if it were not the first time a double petticoat was made. In any case, Brookfield obviously deemed it expedient to acquire rights to both of those patents. Millions of CD 145 insulators were so marked for a number of years. 

CD 145 was the standard telegraph insulator until 1911,whenCD 152 was adopted by Western Union. However, Postal Telegraph, railroads and pipeline companies continued using variations of the CD 145. They were supplied by Brookfield until the plant was closed following the winter of 1921-22.

We do not know when the first CD 145's were made at Hemingray's Covington plant. All things being considered, it is reasonable to assume that the H.G.CO. PETTICOAT units appeared not long after Brookfield began making that style. Insulators are crude, their style is somewhat different than that covered by the Oakman design patent, yet close enough to attract some of the same buyers. Those early H.G.CO. units enjoyed wide distribution. 

The latest Hemingray CD 145 that I've seen was made in 1943. They are still carried in their 1946 catalog. Those latest ones were bought by the railroads.

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Typically how long after a patent date first appeared on an insulator did the date continue to be shown on that style of insulator? (Example: If a Brookfield CD 145 shows the Feb. 22 1870 date, it can be assumed it was actually produced between 1870 and what year?)

Arriving at the age of an insulator by the patent date it carries is impossible. It is obviously true that the insulator was made after the date of the patent; but generally, molds were used until they wore out. There would be no reason to go to the expense of removing a patent date from the mold as soon as it expired. If the patent were issued for a term of 14 years, then renewed for another term, several mold sets would be worn out during that period if the insulator were a popular style. However, in the case of obscure styles that were seldom made, old molds could well be used some time after the patent referred to had expired.

The various markings on the insulators that are so interesting to us collectors were trivial compared to the pressure of production, of providing a product of suitable quality and in sufficient quantity to meet deadlines imposed by major buyers.

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Is there any accurate way to date the manufacturing period of Hemingray insulators by the style of their drip points?

There are approximate time periods that are indicated by the drip points on Hemingray insulators although, as with the patent date markings, it is possible that an old mold might be used at any time it was needed.