1. The Earliest Magnetic Hardness Tester
The earliest Magnestic hardness tester is the Electromagnetic hardness tester published by American patented 2693698 in 1945(As shown in the following picture).
There are a lifting handle, a pressure hand wheel, a hub wheel used for reading Rockwell hardness value, a dial head used to indicating force value, a electronic coil in the magnetizer, and a power socket used to connecting alternating current power.
The biggest problem of this instrument is that it needs the alternacting current power. Obviously, it is quite inconvenient to bring the alternating current power to use the hardness tester on site. Besides, there is hidden danger for personal safety. This instruments only appear on patent literature, the product information can't be found so far.
2. Electromagnetic Hardness Tester Produced in Austria
Austria emco Company once produced a kind of Electromagnetic hardness tester as shown in the following picture.
This instrument is made of three parts, including test head of C-Shape portable Rockwell hardness tester, a small magnetic crane saled in city and a install frame with handle. The writer once had a chance to see further about the emco Company's C-Shape portable Rockwell hardness tester. The test force is applied by two compressed springs, in which one spring produces 10kg initial test force and the other spring produces 150kg or 100kg total test force. The test procedure is that pull the test rod to position 1, the initial test force is pressed on principal axis of test head, then pull the test rod to position 2, apply the total test force, and pull the test rod back to position 1 to initial test force, read the hardness value on the dial head.
It is very convenient to use this instrument to test, except its weight is a little bit heavy. So far, this instrument has been discontinued. The product can not be found in website of enco company and its product catalogue. One thing we can find is a picture of the instrument.