Identifying Screw Bases

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HARRINGTON LIGHTS  is pleased to
provide the following help for 


(Also see:  Identifying Light Bulbs)
*The following method, while tastefully done, has been superceded by a technically superior method.

We know very well that trying to identify a halogen bulb, for instance, that has a small threaded base can be quite a puzzle.  Where and how does one measure?  How precise does the measuring instrument have to be?  How much experience in measuring does one need in order to be proficient in declaring the correct diameter of the base?  Finally, the most disturbing question of all: Is the nominal diameter of the base a reference to the base of the bulb or to the socket in which it fits? 

HARRINGTON LIGHTS has developed the following field expedient method for identifying these small screw bases and the equipment list below is sufficient to allow correct identification of any of three common bases:

Equipment needed:

US coins:  (2) dimes; (1) penny

Canned Goods with extruded* can: (2) Cans
                            *bottom & sides of can is formed by a single metal sheet

Flat, Smooth Surface: (1)

Using the coins as spacers, they may be placed as shown below in order to create a gauge between the tops of the cans that will allow identification of the mystery bulb:

First, place the two cans side by side with one dime in front and the other in back of the cans as shown (click image to enlarge): 

  gauge2.jpg (97268 bytes)   Then carefully push the dimes toward each other until the two dimes touch, as shown (click image to enlarge):

gauge1.jpg (89575 bytes)

(While this serves as a very good gauge, you will find that it has other applications as well.)

The gauge shown above will measure a candelabra base between the top rims of the cans without appreciable "wiggle" ; and this is an E12 (12mm), US Candelabra base.  

Mini-Can (E11) base:  is smaller than the gauge by the thickness of the penny!

E14 European base:  is close to the gauge size, but will not pass!


NEW technically superior method:

Suggested equipment:  

  • Scissors
  • Ordinary 20 lb. copier paper
  • Scale (a ruler)
  • Pen or other marking device with a reasonably small point
Cut a strip of paper about 1/2"x 2".  Wrap the paper snugly around the base of the bulb (holding the first end with a fingernail)  until it overlaps and mark a short line on the paper, starting from the "top" of the overlapping strip to the "bottom" piece, making sure that there is no unmarked gap exactly at the end of the overlap.  Remove and flatten the paper strip and measure from the end of the paper at the mark to the farther end of the other mark.

Interpreting the result:

1-13/32" (between 1-3/8" and 1-7/16") =  E11, Minican base

1-1/2"  =  E12, Candelabra base

1-11/16" =  E14, European base

If you note the above progression, it is likely that an Intermediate base, E17, will produce a measurement of 1-31/32"; and for practical purposes, your 2" long paper strip will be about the right length without overlapping enough to be marked.


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