So no a resistance its not a constant current load....
since voltage is part of the I = E/R
as the voltage increases, to keep a constant current the resistance needs to also increase.
this is a good use of a constant current load... but there not cheap...
so then comes the question... how to load test a supply without one of those fancy smancy constant current loads....
you use different resistor values for different voltage levels that meet your test current requirements.
what are those requirements... the rule of thumb I use is to test within 20% of maximum. (prevents over stressing your transformer, regulator etc)
Here is what I would do personally.
assuming max rating of the 15vac transformer is 1100ma
I would load it to 80% as my max working current (20% safety margin)
so 1100ma * .8 = 880ma max
12v / 880ma = 13.64 ohms.
power = 880ma * 12v = 10.56w * 1.2 (saftey margin) = 12.7w
13.64 ohms is a odd value, so id do a little rounding id use nothing lower then 14 ohms with a min power disputation of 13watt 12 v.
@12v 14 ohms 13watts
10 ohms @ 10 watts would draw too much current and would be under rated for power dissipation at 12v.
at the low end 1.25v..
1.25v / 880ma = 1.42 ohms
1.25 * 880ma = 1.1watts * 1.2 = 1.32 watts
so at 1.25v
1.42 ohms at a min of 1.32 watts. (id likely round up on the resistance to a value I could easily make up or buy and use a 2watt resistor)
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if all you have is a 10 ohm resistor at 10w... and you want to use that to load your supply, then don't exceed lets say 8w (that 20% margin again)
so the max voltage id run to load the supply would be...
8w / 880ma = 9.09v
so lets call it 9V
but your current draw will be much lower then maximum at 1.25v .... 125ma
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if you want an off the shelf resistor you can test though the whole range...
get a 15 ohm 15w resistor (3 series 5ohm 5w resistors will do)
but again the maximum current will be drawn at the highest voltage setting.
It very much depends what your goals are in testing the supply in choosing how your going to load it.
as others have recommended, light bulbs can also be a good cheap load but, be carful there not drawing to much current.
I hope this helps