The development of threaded pinhole insulators is a history of both man's technological tribulations and ingenuity. In the same manner that the actual electromagnetic telegraph and its operating apparatus was continually upgraded and refined, so was the parallel development of an essential component: The perfect insulated wire securing device.

The earliest form of insulators (1844) were comprised of glass plates surround a shellac-laden cloth holding a wire within a notched crossarm. By 1845 a glass insulator similar in shape to a bureau knob was used with unsatisfactory results. In 1846 a center notched glass block was being used in a rapidly expanding telegraph industry. The next major development was the concept of suspending the wire using an insulated iron hook. By 1850 industry requirements for a more successful insulator resulted in continual new styles and wire securing concepts, most notably was an insulator with a smooth cylindrical and tapered pin hole which was then jammed onto a compatible wooden peg. The earliest of this design is referred to as an "egg" insulator, and there were many other threadless styles soon to follow.

But without question, the greatest developmental concept for a insulator was in 1865 when an internal screw thread was patented by Louis Cauvet. Since 1870, mass production of threaded pinhole insulators began and the industry never looked back. Despite the probable costs of insulator manufacturing companies buying in for the rights to use this patented threading concept, they had little choice as it was obvious to all that it was the only proper road to follow.

In Canada, one of the earliest threaded styles is a telegraph insulator known in the hobby as the CD 143. Of the many sub-varieties in this style, there is an insulator that is truly unique within the world of insulators, the double-threaded insulator. 

In North America, the standard for general utility or "acme" threads such as those found on insulators, is four threads to the inch. This thread is formed by one continuous ridge starting at the top of the mandrel and pinhole crown and winding down towards the base at a 10 degree helix angle (Drawing 1.) The "acme" thread is sometimes called a "knuckle" thread. Some of the earliest forms of "acme" threads were actually made in a "V" or sawtooth -- shape up until 1880 or so. The machined mandrel with the threads and the resultant pinhole is always tapered so that the threads fit into each other by compression or interference. A true cylindrical mandrel and pinhole would result in no compression contact until the pin reached the top of the insulator and this could lead to a crown stress fracture. 

The doubled threaded insulator also has four threads to the inch but instead of one continuous ridge it has two. This is called a multiple start thread. The double ridge is achieved by starting each ridge 180 degrees opposite each other and winding each ridge down the mandrel and thus the pinhole. Because both ridges must fit within the same given area, the "angle of the helix" must be double that of the standard helix or 20 degrees (Drawing #2). When a double threaded insulator is screwed into a standard pin, it will still hold tight but will cross thread on the first turn. 

It is not known why or who came up with the idea of double threading insulators but there are some advantages as follows. First and foremost is that the lineman can screw the insulator onto the pin twice as quickly since with each revolution the insulator moves down twice as far on the pin. The second advantage is since there is twice the thread surface, if follows that there is greater compression or interference contact with each thread and thus increasing the holding power.