Re: free standing coil

To: tesla-at-grendel.objinc-dot-com
Subject: Re: free standing coil
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Fri, 13 Oct 1995 07:42:35 +1200
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Steve Roys wrote....
> 
> How do you measure the current into your coil?  

and again....

> As an alternative, and since I like playing with my o'scope, I 
> was going to hook up a resistor in series with my signal 
> generator feeding the coil base and measure the voltage across it 
> to get the current (i=v/r).  Any comments on this technique, 
> either?

    The best way I have found of doing the measurement in question 
is to suspend a scope probe some distance away from the top of the 
secondary coil to minimize loading it. A problem with using resistors
etc in the feed is that it adds an impedance to the coil that causes
its Q to change. IMO even the signal generator source impedance should 
be taken into account for a near accurate figure. All you really need
is to find the peak, set the generator/scope to give full scale 
deflection, then tune either side for 70% of this level to give f1
and f2. High Q coils are extremely sensitive to loading of any kind.
    One of the problems I see with this whole exercise is the need
to standardize the measurement technique or the results are going
to be inaccurate at best, and useless at worst. Perhaps we should
agree on what to do before launching into it ?

Malcolm