Recording Information On Insulators

Reprinted from "INSULATORS - Crown Jewels of the Wire", July 1981, page 6

Very good drawings of insulators can be made through the mail with the use of a "shadow profile" and carefully measured dimensions as follows: 

Place the insulator over a sheet of paper on the floor directly below a ceiling light fixture. Plumb the true vertical with a length of string. Unscrew all but one bulb in the fixture to eliminate multiple shadows. Prop up the top of the insulator so its axis will remain parallel with the floor without having to hold it.

Now get down on your tummy and carefully, trace onto the paper the shadow cast by the insulator. This should look like Figure A below. Don't tweak up the lines on this profile or add any other detail to it (dimensions., hidden lines, etc.) 

Now make a separate sketch of the insulator as shown in Figure B below. Sketch onto it the hidden lines not shown by the profile. Carefully measure the dimensions needed for us to complete the drawing, and add these to the Figure B sketch as shown. Also measure the overall insulator height, the overall base diameter, and the diameter at some indicated point near the insulator crown. (The drawings below are reduced in size to fit this instruction sheet.) 

If the insulator has a saddle top or other details which don't show up in the shadow profile, try to sketch those details on a separate sketch to show what they look like. Some oddball insulators with protruding pieces may also require an additional shadow profile looking down from the top or a shadow profile viewing the insulator rotated 1/4-turn on its axis. 

Lacking calipers, outside diameters can be measured by placing the insulator between two stacks of books and then measuring that distance. The depths of pinhole and recesses between petticoats can be measured by using a narrow strip of cardboard and then measuring its penetration. Measure or estimate the pinhole diameter (the minor thread diameter near the threads collar). 

Thread pitch (threads per inch) is helpful in attributing foreign insulators, since a given country usually uses one thread size throughout. England and related countries use English-unit threads (threads per inch). Most other European countries use metric threads (millimeters per thread). You can determine the exact thread type and its pitch as follows: 

Smooth a small piece of new kitchen foil by rubbing it with your finger on a flat surface. For large pinholes, cut a 1/2" foil strip and insert it into pinhole. Press foil against thread with your finger. For small, deep pinholes,, insert a 4" foil strip and then on top of that a 3/4"strip of innertube or other rubber (tweezers help here). Insert a round pencil on top of the rubber and press hard on the rubber while rolling the pencil and slightly rocking it back and forth. 

Measure from the foil the threads per inch (or twice the number per 1/2 inch). If that doesn't come out to a whole number, measure thread-to-thread in millimeters (or for 3 threads and divide by 3). If metric threads, this should come out a whole number. 

Copy all the insulator markings as best you can, and indicate location of markings on the insulator. On porcelains, indicate type of marking: incuse, underglaze ink, or sandblast. Good foil impressions of incuse markings can be made by repeatedly pressing smoothed foil hard with a rubber eraser. Dry a puddle of glue on the back for mailing.