Hi Malted,
That's not how it works I'm afraid.
The 'constant' is the resistance of the element not the current it draws. An element that draws 15A at 240V has a power rating of 3600W... from a combination of Ohms Law (V=I*R) & the Power Law (P=I*V) we can calculate the resistance as 16 ohms, which is - more-or-less - constant.
So say you only have a 10A max supply, you can then calculate the maximum supply voltage to the element so as not to exceed that current.
I=10A
R=16Ω
V= what we need to know
V=10*16 = 160V
So with an AC voltmeter across the element terminals you wind up your do-hicky until you reach 160V. Note the position of your do-hicky knob as this is the maximum setting for that specific element not to exceed 10A current draw from your socket/supply. You'll be using 1600W btw.
As a practical example... I have ordered 2 x 2kW elements for my keggle. In the UK our max current is 13A per supply ring and we're still basically running @ 240V despite the Euro-bollocks. So I could connect each element to a separate supply, say, one on the dedicated leccy cooker ring and the other on the down-stairs sockets ring and I would have a total 4kW of heating power. Or, I could connect one element direct to my 13A supply (it will draw approx. 8.3A) and the other thro a do-hicky - on the same supply - and limit it to 4.7A. Combined they will draw the 13A maximum.
The upshot being that I can have either 2kW (one element connected), 3kw (one element direct & one variable) or 4kW (both elements connected on separate supplies) depending upon how much heating power I require for batch size, boil control, whatever.
Caveat: This is all in theory. Don't mess with stuff you don't understand especially if it can lead to your death or your house burning down. Always consult a qualified person, etc.
Edit: to add warning.
