Finished the Magneto Current Tester (Ampere meter). There has been past discussion about measuring the voltage and amperage out put of the magneto. The main interest that I have is amperage at cranking speed. According to the notes from Ford Service Bulletin for 1919 at 200 rpm the output voltage is 5-volt AC and 6.1 amperes AC. Measuring voltage is relative easy, but to measure amperage one needs something similar to the St Louis Electrical Works magneto tester. The voltage can be measured with any AC voltmeter. But measuring the amperage has been a challenge. Tom Carnegie published "A Poor Man's Mag Tester" based on the St Louis Electrical Works magneto tester. I was looking for a more period instrument.
I built an amperage tester based on a swap met 10 Amp Jewell Portable AC Meter ($20.00) and a coil core from Snyders ($19.25). In the diagram found in Dykes the St Louis meter used a "reactance coil" to provide a constant current. So why did I decide to use the coil from Snyders as a reactance coil? According to a 1920's publication, Automobile Catechism, a reactance coil is a kick coil. And the kick coil is a kind of spark-coil used for low-tension (make and break) ignition when electrical current is supplied by a battery. Based on the description found in the text the coil from Snyders is a reactance coil.
I have provided a diagram of the finished circuit. The black box contains both a one-ohm resister and the reactance coil. Coil has a resistance of .7 ohms. The meter with shut is for 10 amperes ac.
Looks good, but, does it work?
How are you able to crank the engine at 200 RPM?
Will the starter crank it that fast?
I'll be testing it this coming week. If the data is correct with the engine running at idle (200 rpm) there should be a reading of 6.1 amps. From the 1926 Ford Service Bulletin hand cranking should give 3.5 amps AC. When hand cranking, it may help to remove the spark plugs. This is to be seen.