Potheads and Spur Insulators

Collectors of foreign porcelains seem to be generally excited about finding the peculiar shaped insulator nicknamed the "Pothead". It has a top cover which unscrews from the rest of the insulator and resembles a jar of peanut butter with a lid. This insulator design was invented by Colonel Sir T. F. Purves and J. Sinnott of London, England, covered by U.S. Patent #898,921, September 15, 1908, and first introduced at that time in England. English manufacturers were Bullers Ltd. (London) and Taylor Tunnicliff and Co. (Hanley). 

In the U.S. we would call this design a "dry spot" insulator. In England it is referred to as a terminal insulator. Quoting from Worldwide Porcelain Insulators, co-authored by Jack H. Tod and myself, "Styles vary as to one or two holes for lead in wires, etc. The purpose of these insulators is to provide a "dry spot" on the insulation of the lead-in wire, thereby breaking the wet weather shorting path from the line wire over the insulation of the lead-in wire to ground." It provides the cheapest and most efficient method which has ever been devised of connecting aerial wires to covered leads.

Quoting now from some xeroxed material sent to me by Alex Burnett (Vermillion, Alberta, Canada), which I believe was taken from a book called Telephony (author ?), "The connection between the open wire and the covered lead is made in a cavity formed at the top of the insulator; this cavity is then filled with insulating compound and covered by a screw cap. In this way a lead-covered cable may be carried up to the insulator, where the end is effectively sealed by the compound. Telephone subscribers' circuits are led in with lead-covered cable having 2-wire enameled and F.P.-covered 10 lb. conductors. 

"The insulating compound is soft enough to be pressed cold into the insulator top and is not materially affected in consistency by changes of temperature met with in practice. It has a high insulation resistance when dry, and this is maintained when in contact with water. Its composition is by weight: 56% French chalk (magnesium silicate), 32% Palm pitch and 12% coal tar.

"The compound should be thoroughly pressed into the cavities of the insulators so as completely to enclose the bare wire joints, care being taken to see that the cavities are quite dry before it is put in."

A couple of my pothead insulators from England do have this strange putty-like substance in the top cavity, and it was so interesting to me to learn what it was made of and why it was there!

These insulators were made of white glazed porcelain or black composition. Depending on the size of the insulator, the small threaded pinholes were made to screw directly onto either a 5/8" or a 3/4" steel spindle (pin). 

One other manufacturer of the "pothead" insulator was Sunshine Porcelain Potteries of Melbourne, Victoria, Australia. According to information coming from Laura Van der Endt and Dawn Hole, who both live in Australia, this company sold to a switchgear manufacturer which eventually ceased all manufacture of porcelain products. Apparently other styles of insulators had been made as well. I have no other information about Sunshine Potteries, i.e., history, dates, etc. However, the pothead insulator I have with their trademark is dated 1934. 

A Swiss company known as Suisse Langenthal also produced the "pothead" insulator, as well as many other styles of insulators, and is possibly still in business. We have no other information about this company, not even where in Switzerland it was located. Can you help? Specimens we borrowed from Steve Watkins (Portland, Oregon) and Joe Tuma (East Islip, New York) for drawings were dated 1953 and 1954 respectively. 

The picture below shows several variations of the "pothead" insulator. 

Viewed from left to right we see examples of the following manufacturers: 

1. Taylor Tunnicliff and Co.; incuse trademark on top of lid; patent dates on skirt. (A new variation and as yet no U-number assigned.) 
2. Sunshine Potteries; incuse trademark on lid: This is U-1938. 
3. Taylor, Tunnicliff and Co.; known as a terminal "spur" insulator. This is U-1944.
4. Unidentified English manufacturer; incuse letters G.P.O. (General Post Office) plus a "hand" trademark on the skirt. This is U-1941, known as a double groove "pothead".
5. Suisse Langenthal -- trademark stamped in green ink up inside skirt. (New variation, not yet classified.). Insulator borrowed from Steve Watkins.

Following is a picture of several variations of the "pothead" in black composition.

Again reviewed from left to right: 

1. Manufacturer unknown; raised letters G.P.O. on Lid; probably English.
2. Manufacturer unknown; raised figures on lid are: NO 16 -- L R; found in New Zealand, but probably English.
3. Manufacturer unknown; marked: NZPO 69 (New Zealand Post Office); double groove "pothead" made of hard rubber -- extremely light weight; found in New Zealand.
4. Manufacturer unknown; raised letters: LEA NZPO 62; double groove "pothead"; found in New Zealand.

Spur insulators:

One particular design of "pothead" had a side knob, and in the first picture I showed you it is #3, and is referred to as a "terminating spur insulator". It is the white porcelain insulator right in the center. It came into use some time between 1912 and 1920. The British Post Office referred to it as their insulator #21. Still another British design of spur insulators is their #10, described as "a small double groove, double shed, Sinclair (patent name) insulator with side knob for a 5/8" "spindle". It was introduced before 1907 and was at first available in either white porcelain or black composition. It was not until sometime between 1946 and 1955 that brown porcelain was added to the list. The example shown in the picture below is our U-1578, and it is brown! 

Again quoting from our Worldwide Porcelain Insulators book, "Spur insulators are for applications where the direction of a wire abruptly changes from a horizontal to a vertical route. Also similarly, when a tap line is taken off the through line either vertically or nearly so. The reason this side knob also appears on some designs of "dry spot" (pothead) insulators is that the lead-in wires nearly always run vertically."