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  • 19 Jul 2010
      Insulators, what are they, what were they used for, what were they made of, and are they of any value? Glass insulators were first produced in the 1850's for use with telegraph lines. As technology developed insulators were needed for telephone lines, electric power lines, and other applications such as railroad signals. In the mid 1960's a few people began collecting these antique insulators. Today there are over 2000 collectors. Production for porcelain insulators started in the 1850’s and 60’s. Porcelain insulators were more commonly used for power distribution due to their greater strength and surface resistance; by 1915 they basically replaced all glass insulators on electrical lines. Popularity has increased for porcelain insulators over the past ten years thanks to the rise in glass prices, earlier ‘classic’ porcelain being taken out of service as utility distribution voltage increases and the increase of available information on the web and in books. What are insulators? According to Webster, to insulate means "to separate or cover with a nonconducting material in order to prevent the passage or leakage of electricity, heat, or sound”, in order to keep electrical and communication lines dry and suspended above the ground they were supported by poles, something was needed to keep the lines off the wet poles, along came the insulator. There are many different types of insulators and patents for even more. Insulators are grouped by material, origin, number of parts and threading characteristics. Origin is either North America, where the predominate material for insulators has always been glass for communications and porcelain for electrical or foreign, some countries preferred and used only porcelain even in telegraph lines. The number of parts is separated into two categories unipart, consisting of one solid piece, or multipart, any insulator of two or more separately molded parts, either cemented together during or after manufacture, or used together on the line. Another characteristic being threaded, an insulator with internal screw threads which correspond to matching threads on a pin, or threadless, an insulator without threads and having a smooth pinhole; there are also non-pintype insulators, which include spools, dead end insulators, and wire strain insulators. What are they made of and why are they colored? Most commonly, insulators are made of glass, but they are also made from porcelain. In the Smithsonian you can find an insulator dated back to 1872 made of wood, or rubber from 1940. Glass insulators are most commonly found varying from an aqua blue to a green in color. Depending on the type of glass cullet, sand and chemicals used in making them, they can be found in colors from clear to purple, aqua to emerald green, yellow to red-black and many colors in between, to see a range of these colors visit  Whereas glass insulators were colored due to glass cullet, colored porcelain insulators provided markers (Green for series street light circuits, Yellow to denote a power line on a telephone pole, etc.) or to identify different utility company lines or circuits. Porcelain insulators ranged from browns to blues, greens to yellows, ‘butterscotch’ to grey, and even white. Sadly though a multitude of grays, greens, blues, yellows, and cobalt's can’t be found any longer Certain styles common in one color maybe rare in another and therefore more desirable. The brown insulators tend to be less popular unless found in a rare style. Earlier porcelains will have mottled or swirled glazes, when compared to the more recent uniform glazes. Are they worth anything? There are many factors that determine an insulator's value. Shape, color, embossing, condition, desirability and rarity all affect its value. Most insulators are quite common and have little monetary value. The first step in determining your insulator's value is to determine which insulator you have. Even with a price guide in your hand, you have to determine which of the approximately 460 shapes, 2800 different embossings, and almost 9000 color combinations best describes your insulator, and that’s only counting the glass variations. It is easy to list the common insulators, and also list the very rare insulators, but it is difficult to list all the thousands of collectable insulators that fall between those extremes. A slight difference in the shape or color of your insulator can affect the value immensely. This information was gathered from the following sites:, For a condensed history view my blog History of Insulators.
    833 Posted by Christina Errington
151,796 views Jul 19, 2010
Slight History on Insulators

Here is  a Slight history on insulators, pertains mainly to the glass variety, to see a full history please feel free to visit where I found the following facts from.

  • May 24, 1844:  Samuel Morse transmitted the first telegraph message over a short telegraph line run along a railroad between Washington, D.C. and Baltimore with the famous words "What hath god wrought!".  Based on advice from Ezra Cornell, the insulators used on this line were made of two flat glass plates surrounding a cloth wrapped wire in slot in the crossarm.
  • April 18, 1846: Royal E. House of New York, NY patent for a telegraph that prints characters to be decoded titled "The Magnetic Letter Printing Telegraph".  The idea was different enough from Morse's design to avoid infringement.  An unusual metal and glass insulator was developed uniquely for the House telegraph.  (US Patent 4,464)
  • July 24, 1846 Addison Smith of Perrysburg, OH patent for a fire detector and alarm system using telegraph to transmit information.  (US Patent 4,661)
  • April 5, 1848:  Ralph Gray and Robert Hemingray signed a five year lease for a small half-lot on Hammond Street (originally known as  Mayor's Alley from Third to Fourth, between Main and Sycamore) in Cincinnati, Ohio.  They soon began manufacturing glass at this location under the name of Gray & Hemingray Glass Works.
  • February 5, 1850:  James Spratt of Cincinnati, OH patent for a lightning rod insulator design.  This patent was implemented in an early LRI (Lightning Rod Insulator).  (US Patent 7,076)
  • October 14, 1851:  John Yandell of  St. Louis, MO  patent for a glass block insulator.  A good example of this CD 1014 insulator exists in the Smithsonian Institution archives.  (US Patent 8,438) -- [Full Patent Text]
  • March 29, 1859:  Russel Hickok of Fort Edward, NY patent for a glass lightning rod insulator design.  Link for more details! (US Patent 23,373) -- [Full Patent Text]
  • August 5, 1861: The Transcontinental Telegraph, which connected St. Joseph, Missouri and Sacramento, was completed by the Western Union Telegraph Co. and its associates.  In 1869 this line was rerouted to follow the Transcontinental Rail Road
  • July 25, 1865:  This important patent was by Louis Cauvet for the a method of forming internal threads an insulators to allow them to screw onto a threaded pin.  Previous insulators were threadless and held on the pin by friction. Brookfield was the first to license this patent.  (US Patent 48,906) -- [Full Patent Text]
  • December 19, 1871:  Robert Hemingray patent for a technique for molding glass insulators.  This patent was used on a very large variety of insulators.  (US Patent 122,015) -- [Full Patent Text] -- (Additional Patent Image)
  • January 23, 1872:  Chester H. Pond of Cleveland, OH patent for a threaded wooden insulator with a metal cap.  Several excellent examples of these insulators can be found in the Smithsonian archives.  (US Patent 122,961) -- [Full Patent Text]
  • February 6, 1877:  Paul Seiler patent to provide six longitudinal ribs to both strengthen the insulator without adding weight and reduce the contact area with the tie wire.  This patent was implemented in CD 130.2.  (US Patent 187,183) -- [Full Patent Text]
  • January 14, 1879:  James M. Brookfield design patent for the CD 102 "pony" style insulator.  (US Design Patent 10,981) -- [Full Patent Text]
  • September 13, 1881 Samuel Oakman patent for a process of forming threads on an insulator by plunging a previously formed (and cool) threaded glass cup into the molton glass in the mold.  The glass would cool enough as to not overly distort the threaded cup.  This technique was used by the American Insulator Co. on many of their insulators as well as some unembossed pieces and a CD 134 marked with just the patent.  This is the reason that many of these pieces have poorly defined or somewhat distorted threads, as the glass cup would soften in the molton glass.  (US Patent 247,100) -- [Full Patent Text]
  • May 1, 1883 Joseph S. Lewis patent for an external thread above the wire groove to allow the insulator to be "screwed in" to a tie wire.  a damaged insulator could also be replaced without undoing the tie wire.  The patent drawing shows the threading to be in the same direction as a normal insulator's internal threads, making one loosen it on the pin to attach the tie wire (Not a desirable function).  Frank Pope's patent later this same year (Dec. 25, 1883) is for virtually the same design, only threaded in the opposite direction.  Both patents appear on the National Insulator "Corkscrew" styles CD 110.5 and 110.6.  (US Patent 276,839) -- [Full Patent Text]
  • August 14, 1883:  Homer Brooke, of Jersey City, NJ patent for an insulator press.  This patent was implemented on a number of Brooke's insulators including CD 120 and 133.1 and it is likely that this is the "patent applied for" on a number of other styles attributed to Homer Brooke including CD 120.2 and CD 125. Thanks to Bob Stahr for providing this data originating from Dick Roller who put together a great reference data base. (US Patent 283,321) -- [Full Patent Text] -- Link to additional Homer Brooke information.
  • October 16, 1883 Bradley A. Fiske and Samuel D. Mott patent for diamond shaped indentations in the wire groove to reduce electrical contact with the tie wire.  This patent was implemented in both CD 135 and CD 109.  (US Patent 286,801) -- [Full Patent Text]
  • December 25, 1883:  Frank L. Pope patent for external threads opposed to the internal to allow the replacement of a broken insulator without disturbing the tie wire.  This patent was implemented in CD 110.5 and CD 110.6. (US Patent 290,922) -- [Full Patent Text]
  • June 24, 1886:  The pottery & Glassware Reporter was informed by Mr. E. A. Leonard that the Leonard Glass Works of Detroit, Mich., has closed down and may not be reopened
  • August 17, 1886:   John O'Brian of New York, NY patent for a unique insulator design assigned to William Brookfield.   This patent was implemented in the Brookfield CD 119 insulators, although the embossing provides the wrong patent date.   (US Patent 347,635) -- [Full Patent Text]
  • November 23, 1886:  Robert G. Brown of Brooklyn, NY assigned to the E.S.Greeley & Co. patent for an insulator and pin to allow mounting below the crossarm.  This design could double the number of circuits supported by a single crossarm or be used to facilitate wire transposition.  This patent style is known as the "Brown Pony" and was implemented in CD 187 & CD 188 as well as U-81, U-82, U-84 and U-85.  (US Patent 353,120) -- [Full Patent Text]
  • November 8, 1887:  Francis H. Soden and Henry Goehst patent for a strain insulator for electric lights.  This patent was implemented in the CD 1129 glass strain. (US Patent 372,940) -- [Full Patent Text]
  • June 17, 1890:  Samuel Oakmen patent for the ears found on cable insulators - CD 258, CD 259, and CD 260 match closely the patent drawings.  (US Patent 430,296) -- [Full Patent Text]
  • August 19, 1890 Samuel Oakman for a skirt projection to act as a water stop as well as threading the inside of the inner skirt to increase the leakage distance.  This was implemented in CD 258, CD 259, and CD 260.  (US Patent 434,879) -- [Full Patent Text]
  • December 23, 1890:  Foree Bain patent for grooves inside and outside the insulator surface for the dual purpose of increasing leakage distance.   This has been implemented on CD 144.  This patent was assigned to the Hemingray Glass Company on February 28th, 1901. (US Patent 443,187) -- [Full Patent Text]
  • February 1891:  Pass and Seymour of Syracuse, NY starts making threaded porcelain "wet process" insulators.  The only known existing style they made is U-146, although they also catalogued U-141.  Sometime in 1895 or 1896 they stopped making pintype insulators.
  • May 2, 1893: Ralph G. Hemingray patented drip points.  The intent was to provide a point for moisture to accumulate and more quickly drip off the insulator keeping it dryer.  This date is considered significant as drip points were so widely implemented.  (US Patent 496,652) -- [Full Patent Text]
  • August 29, 1893:  George W. Blackburn of Palmyra, NJ patented an insulator design using a bail clamp to hold the conductor wire.  This patent was implemented in CD 141.6 (US Patent 504,059) -- [Full Patent Text]
  • August 14, 1894:  George H. Winslow of Pittsburgh Pennsylvania patent for a unique series of oil cup insulators for high voltage use.  This patent appears to have been implemented in several rare insulator styles including  CD 180, CD 180.1 and CD 244.  (US Patent 524,659) -- [Full Patent Text] -- (Additional Patent Image).
  • September 25, 1894:  David N. Osyor patent for a pin and insulator combination that was assigned to the Jeffery Mfg. Co.  This was implemented in the CD 185 Mine insulator and porcelain U-89 through U-98B.  (US Patent 526,498) -- [Full Patent Text]
  • 1895: The first underground trolley system in the United States is constructed in Boston, MA.  This system used relatively low voltage DC which required very large copper cables.  The CD 140 "Jumbo" was designed for this line.
  • September 3, 1895:  Danial Rothenberger patent for a unique cable style insulator with a hole through the crown perpendicular to the tie wire.  This patent was implemented in the rare Brookfield CD 268.  (US Patent 545,819) -- [Full Patent Text]
  • February 12, 1896: "China Glass & Lamps" reports the new glass works being erected at Westport, Md., by the Baltimore Glass Mfg. Co., is nearing completion.  Later reports show production started by the end of March making Screw cap ware, fruit jars and electrical supplies. -- Their insulators are marked B.G.M.Co. and are mostly found in purple glass.
  • April 7, 1896:  Hannibal W. Rappleye patent for a bail-clamp tie arrangement to hold the conductor.  This patent was implemented by Brookfield in the rare CD 134.6 (US Patent 557,881) -- [Full Patent Text]
  • September 28, 1897:  Fred Locke patent for a power insulator with an oblong shape and side troughs to direct water away from splashing on the crossarm.  This patent was implemented in the U-937 insulators that Locke had Imperial Porcelain make for use on the Niagara to Buffalo power line.  (US Patent 590,806) -- [Full Patent Text]
  • March 8, 1898:  John W. Boch patent for a three piece porcelain power insulator where the three shells were fused together with extra glaze.  This was implemented in the Classic Thomas styles U-928 and U-928A.  (US Patent 600,475) -- [Full Patent Text]
  • June 7, 1898:  Ralph D. Mershon of Colorado patent covering a power insulator design with a far extended inner petticoat and ridges on the top skirt to direct water off the insulator.  This patent was implemented in CD 288 and CD 298 as well as U-938, U-944 and U-945.  (US Patent 605,256) -- [Full Patent Text]
  • September 3, 1898: A British patent by Daniel Sinclair and William Aitken both of Oxford Court, London patent for a two part dry spot insulator implemented in U-1925,U-1929, and U-1929A. (UK Patent 25,816 of 1897) -- [Full Patent Text]
  • September 19, 1899:  Frederick H. Withycombe of Montreal, Canada patent  for a various ridge designs on the outside of an insulator to provide a "cushion" to damage from projectiles (ie: thrown rocks).  This first patent illustrates horizontal ridges.  He released four very similar Letters Patents and two design patents for virtually the same ideas.    Horizontal ridges are found on a number of Canadian CD 143's.  (US Patent 633,173) -- [Full Patent Text]
  • 1900:  An eight mile elevated section of Boston's transit system was constructed.  Heavy DC cables were required and this line used CD 267 and CD 267.5 insulators without tie wires.  The weight of the cable was sufficient to hold it in place below  the tracks.
  • June 10, 1902 Vernon G. Converse patent for a stacking insulator.  This patent was implemented in the amazing glass insulator comprising CD 317.8, two CD 313 sleeves, and one CD 313.1 sleeve.  (US Patent 701,847) -- [Full Patent Text]
  • April 7, 1903 Ferdinand W. Gregory of New York, NY patent for a square wire groove providing extra support for the conductor.  This was implemented on the scarce Brookfield CD 159.    (US Patent 724,848) -- [Full Patent Text]
  • May 19, 1903:  Fred M. Locke patent for the design of the M-2795 insulator.  (US Patent 728,805) -- [Full Patent Text]
  • November 17, 1903:  Edward F. Schoethaler of Longbranch, NJ patent for a unique insulator design.  The drawings look very similar to the recently found "Spaceman" CD - The intent was to provide extra protection from a wire coming undone which may indicate that this idea influenced  the rare CD 139 Brookfield "Combination Safety" insulator.  (US Patent 744,631) -- [Full Patent Text]
  • April 26, 1904: Scott Cutter patents the unusual CD 1038 glass Cutter tree insulator. (US Patent 758,175) -- [Full Patent Text]
  • March 24, 1908:  Leonard W. Storror of San Francisco, CA patent for an insulator with an insert to improve insulation by making a better barrier to moisture.  This was implemented in the Brookfield CD 211 "No Leak" insulator.  (US Patent 882,803) -- [Full Patent Text]
  • April 6, 1909: Charles E. Eveleth of Schenectady, NY patent for a porcelain power insulators with skirt grooves to allow pieces to break off if hit by a projectile preventing the loss of the whole insulator.  This patent was implemented in the rare M-2202 and M-2202A porcelain power insulators.  (US Patent 917,031) -- [Full Patent Text]
  • March 19, 1910: The Hemingray Glass Company received Trade Mark No. 79,096: “HEMINGRAY” for use on ‘electric, telegraph, telephone, cable, street-railway, and floor insulators and break-knobs of glass.’ It was noted that the trade mark had been in use for 10 years.  Link for additional Hemingray information.
  • January 16, 1912:  John Hilliard Jr. and Charles E. Parsons of Glens Falls, NY patent for a unique rigid suspension insulator made with multiple insulating shells mounted on a rod with two metal ends.  This is quite likely the patent for the recently found suspension insulators made from five CD 314 Hemingray shells.  (US Patent 1,015,229) -- [Full Patent Text]
  • August 11, 1914:  Benjiman S. Purkey of Tacoma, WA patent for a twist lock "No Tie" porcelain insulator.  This patent is implemented in U-186.  Although unmarked, the recently discovered CD 207.5 may also have been made to this patent.  (US Patent 1,107,111) -- [Full Patent Text]
  • Sept. 18, 1917:  Louis Fort of Jersey City, N.J. patent for a porcelain and metal two piece clamp insulator for street light drops.  These unusual insulators were made in brown porcelain with a cast metal clamp and mounting. (US Patent 1,240,330) -- [Full Patent Text]
  • February 26, 1929 Rufus Gould of New York, NY patent for a dry spot insulator assigned to the Postal Telegraph Co.  This patent was implemented in Whitall Tatum CD 182 and porcelain styles U-173, U-174, and U-175.  It called for a large inner skirt gap where the drop wire could be potted to keep wetness out.  (US Patent 1,703,853) -- [Full Patent Text]
  • April 9, 1929:  Leon T. Wilson of East Orange, NJ assigned to A.T.& T. CO.  patent for a low loss glass insulator design.  This patent was implemented in Whitall Tatum CD 176 and the recently discovered Hemingray version CD-176.5.  (US Patent 1,708,038) -- [Full Patent Text]
  • June 22, 1937:  Bentley A. Plimpton of Victor, NY patent for a porcelain high voltage insulator with additional petticoats and flanges.  This patent was implemented in the porcelain "Hi-Top" series of insulators (U-782 through U-805) as well as glass styles CD 220 and CD 221.  (US Patent 2,084,866) -- [Full Patent Text]
  • November 16, 1937: Donald H. Smith assigned to the Western Union Telegraph Co. patent for a metal insulator shield to go around the base of a CD 154 style insulator.  I have seen these in use on Canadian dominion CD 154's.  (US Patent 2,099,540) -- [Full Patent Text]
  • July 11, 1939:  H.H. Wheeler assigned to Western Union Telegraph Co.  patent for a low loss telegraph insulator.  This patent was implemented in CD 122.4 by Corning and Hemingray.  Of more interest, this patent covers the carnival glass coating used on many of the telegraph styles including the CD 118, 142 and 142.4.  It states that the coating increased the surface resistance of the insulator, thereby improving its performance in damp weather. (US Patent 2,165,773) -- [Full Patent Text]
  • October 15, 1940 D.H. Smith of the Western Union Telegraph Co. patent for a threaded rubber insulator.  This patent was implemented in the smaller Continental Rubber Works insulators.  (US Patent 2,218,497) -- [Full Patent Text]
  • January 8, 1946:  Alwin G. Steinmayer of Milwaukee, WI patent for a combination insulator and spark gap arrestor.  This patent was assigned to the Line Material Company and was implemented in Hemingray insulator CD 186.1 and CD 186 and CD 186.2 were likely similar experimental pieces.  (US Patent 2,392,342) -- [Full Patent Text]
  • November 30, 1948:  Rogers Case patented a mid-span transposition bracket as used with the Owens Illinois CD 1049 insulators.  (US Patent 2,455,229) -- [Full Patent Text]
  • October 30, 1962:  William F. Markley and James L. Slater patent covering the design of several insulator styles made of rubber.  This patent was assigned to the Western Union Telegraph Co.  Several styles made by the Continental Rubber Works match this patent.  (US Patent 3,061,667) -- [Full Patent Text]