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. 

Please settle a friendly bet between two new collectors. My friend maintains that most small glass insulators not having saddles, commonly called telephone, telegraph or railroad insulators, were strictly used by telephone or telegraph operations, and never by electric utilities. I argue that these same insulators were also used by some electric utilities many years ago for stringing light-duty service wires and small rural lines.

I don't believe that every so-called telephone insulator was originally designed strictly for telephone use, but rather for any application needing light-duty insulators. I also maintain that the labels "telephone", "telegraph", and "railroad" insulators stuck because that just may have been their predominant use in a certain region. Some valued comment, please, for there is a lot riding on this bet!

Jim Sovaiko, one of the two Pennsylvania collectors! 

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In all but perhaps a very few cases, insulators were designed for a specific use, and were purchased for that purpose. However, one cannot say they were never used in another type of service. When linemen run out of insulators, they will latch onto whatever is available. During World War II, many insulators of various types were pressed into service in unusual situations. Some still remain there today. And when the hundreds of private telephone systems were composed primarily of open wire lines, any insulator that would fit the pin or bracket was often used. 

In the case of new insulator purchases, however, it would be most unusual to find a design intended for one type service being bought for a different one. Rather, as the need for new applications arose, new styles were designed for them.

For example, CD 134 was designed for the early DC lighting circuits. Its was patterned after the CD 133 telegraph insulator, but with a deeper groove for the heavier wires used. CD 133 was never sold for anything other than communication work; CD 134 went for DC lighting and for railway signal circuits. 

The familiar CD 102 was simply a small version of CD 126, the standard telegraph style at the time telephone lines were adequate for the smaller gauge wire and short spans used in telephone work. A few years after, as long distance lines began to extend from city to city, CD 121 was developed for that service. It, in turn was never used in telegraph service and by that time CD 145 had become the standard telegraph style and, in turn, was never sold for telephone service.

Exceptions might be CD 160 and CD 162. Both of these were originally for low voltage electric service. Later, CD 160 was sold entirely for telephone work; and CD 162 was used for railway block signal and sometimes communication lines, in addition to its primary use as a secondary distribution insulator.

In summary, insulators were indeed designed and sold for very specific uses. However, as the years passed they did get switched around and one cannot say that an insulator was never used in an application other than that intended by its manufacturer and designer.

From Bill Meier of Carlisle, Massachusetts...

I have a CD 114 Hemingray No.11 in aqua with S.D.P. (sharp drip points) and the standard embossing. The front skirt reads HEMINGRAY/No 11 and the rear skirt reads PATENT MAY 2 1893. However, embossed on the crown top is MAY 2.  It is neither centered on the exact top nor lined up with either mold line. Anyway, the mold line stops at a point about 1/8" above the highest groove -- a standard three piece mold, I assume. The MAY2 embossing is definitely raised.

The "MAY 2" on the crown of the Hemingray No.11 resulted from a rather common occurrence during the manufacture of the old insulators on hand presses or even the early semi-automatics. As you know, molten glass is not really a liquid, but rather like the consistency of honey. As the glass was fed from the furnace for delivery to the individual molds, it was sheared off manually a blob at a time and fell into the mold. It would not always fall exactly in the same way, and not always exactly in the center of the mold. Often, if the mold was a little cooler than it should be, the blob of glass would strike a spot on the engraving as it fell, and in that fraction of a second would cool enough to retain the impression in the glass as it folded down into its final position in the mold. It is quite common to see such embossing on the skirt of the insulator, just above the embossing that was intended. It is less common for it to be all the way on the top of the insulator; but it happens in the same way.

I have a CD 124 Hemingray insulator embossed HEMINGRAY/No 13. There is a small (about 5/8" in diameter) slightly raised circle around the 13, and it is extremely clear that a "4" is between the "1 " and "3". I also have the same insulator embossed HEMINGRAY/ No 4.

The other difference is in the molding of the threads . The No.4 threads are 2-1/4" deep coming to within 5/8" of the bottom of the skirt. Since the skirt is wider at the bottom, the threads end with a 1/4" flat ridge.

The No.13 threads are only 1-1/2" deep ending in a fairly smooth transition into the skirt with less than a 1/8" flat ridge. And they start 3/8" down from the crown.

Hemingray No.4 was re-numbered 13 sometime around 1915. Several other styles were renumbered about the same time. In my 1911 catalog, the CD 124 is shown as No.4; and in both the 1918 and 1921 (the latest reference I have to that style) it is catalogued as No.13. Since it was not a popular style, and especially that late in its history, not many were made, so there are not many No. 13 Hemingrays around. Perhaps they are not rare, but certainly scarce. The same is true of the old W.U. No.5 which was renumbered No. 15 about the same time. 

The differences you describe in pinhole depth and skirt length are not due to different numbers, but are differences in those particular pieces. On those old Hemingray hand presses, the mandrel and collar were both free to move vertically and position themselves according to the exact amount of glass in the mold. If less glass was sheared off into the mold, the mandrel would be lowered farther and the crown of the insulator would be thin. Also the collar (which was spring loaded) would go farther, making a longer skirt. As the blob of glass was increased in size, the reverse would be true, an insulator with a thicker crown and a shorter skirt. So these are features that vary with individual insulators and depended on how skilled the workers were at getting the product as nearly perfect as possible.

Another insulator with embossing over a No. 4 is a CD 295 Hemingray 72. This over-embossed "4" is neater than on the No.13 and directly under the "7" of "72".

Your Hemingray No.4 re-numbered 72 is about the same situation as the others. Later molds were engraved with the number 72, as a matter of fact that style was made as late as 1931 or 1932. Incidentally, the number 4 was also used on CD 281, which is a side tie unit of the same size and general style as the CD 295. This may have been an error, I haven't seen enough of that particular unit to make a decision.