Apparatus for determining the number of turns of a magnetic coil

An instrument for determining the number of turns of a test coil wound on a magnetic core, comprising:

• a reference core;
• a reference coil wound on said reference core with a predetermined number of turns;
• a first load coupled across said reference coil;
• a second load of adjustable magnitude to be connected across the test coil;
• means to couple the test coil to said instrument;
• said coupling means comprising first means to connect primary winding means linking both of said cores in common;
• said coupling means further including second means to connect the voltage on said test coil to said adjustable load;
• means for supplying a-c current to said primary winding means;
• the magnitudes of said first and second loads being set at values to operate both of said coils as heavily-burdened current transformers;
• comparator means for comparing the a-c signals developed by said two coils;
• and
• means for producing a measurement output corresponding to the value of said adjustable load when it has been set to produce a null between said a-c signals of said two coils.