Versatility of the type 310 meter could be enhanced by several accessories.
Pix 1 shows the Model 100 package, which contains a 310 meter (type 1 shown here) with manual and probes as well as the accessories that add "clamp on" AC amp reading capability to the meter. The important elements that are added to the 310 are the model 10 clamp on probe, which attaches to the top of the 310 meter body, the model 101 adapter for measurement of current through a standard wall outlet (2 prong only) and an extension, for using the clamp probe remotely from the 310 (not shown). A "genuine cowhide" carrying case for the system is included. The Model 100 package sold for $78 (1971 Allied catalog), compared to $44 for the standard 310, $56 for the 310C, and $74 for the 310FET, making it a decent bargain.
With the increased versatility came some confusion... To read the current correctly, the user
had to select the 3VAC scale on the meter. This special scale was somewhat non-linear, due to the characteristics of rectifier diodes. The user
then had to multiply as necessary to determine the correct value for the various ranges provided by the switch on the clamp on attachment (6 to 300 amps).
When the "type 3" 310 meter was introduced, the confusion was somewhat remedied, as more numbers were added below the 3VAC scale, to aid the user in simple multiplication. The designers also clearly marked the 3VAC range switch with the designation "AMPS", which caused another confusion... everybody
knows that one should not try to measure voltage with a VOM set to Amps, so does the 3VAC range still exist?
With the introduction of the "type 4" the confusion was remedied as well as possible (short of adding another switch position just for AC amp measurements)... the added numbers were left in place below the 3VAC scale but the switch position reverted to the earlier 3 VAC marking.
Pix 2, left to right, shows a "type 2"
without the added numbers, a "type 3" with the additional numbers and the "AMPS" switch position and a "type 4" with the confusing switch nomenclature reverting to the original 3VAC designation. All three meters display the red "Triple T" logo, used in the 70s and 80s.
Pix 3 shows several types of model 10 attachment. Left to right, an early model for the 310, a model for the 310C (necessary because the "C" model had different AC sensitivity), both with a bakelite case, and a later model, with a plastic case, and a 2 wire connector, so that the probe could be used (somewhat) remotely from the 310 (or 310C, in this example) without the need for an extension cord.
Provisions for attachment/connection of the type 10 probe changed over the years, as shown in Pix 4. The early 310 (type 1, with red dot) shown on the right, did not have the attachment point for the type 10 probe. It does, however, have the ground plug... more on that later. The later "type 1" in the center of the picture, has the attachment point added. The "type 7" on the left no longer has the ground connection (only a blank hole), necessitating the use of the later 2 wire probe attachment, similar to the one shown in Pix 3.
jr