OK the data for that specific transformer is  here  Oddly that data sheet says it generates DC;  but the diagram on the case implies  the secondary is indeed a proper 50Hz type of device, and there are 'taps' or terminals part way along the secondary coil, so you can either take 24V from the two extreme ends, or 12V from one end and the terminal in the middle,  and as there is a further tapping point from approx 1/3 of the  way along one of the 12V halves, you also have the option of 8V or 4V instead, not useful for your case but it is actually quite a versatile little unit.


Please use your meter to verify the output is AC while we complain to BG that either their data sheet is incorrect or their device labelling...  like they care.


So, assuming that it is the right thing, and  I presume then that you are seeing 10V across the '8V' section of the output winding, as these voltages are all only truly correct supplying the nominal full load, and can be expected to rise a bit off-load, and fall a bit on-load (20-25% variation between on and off load is quite common on small transformers like this - a normal door bell does not give a hoot, and thinner wire keeps the size down).


I suggest you look at the other  pairs of output terminals on the tranny, and select the ends to get  nearer to 24 (or maybe use one end and the middle  '12V' one if it is really more like 16V - 17V, and the '24V setting is a more like 30 ),  then run a pair of wires to the resistor, which  needs to be in-line with one of  the leads, as per the last diagram I linked to in my previous  post.

OK the data for that specific transformer is  here  Oddly that data sheet says it generates DC;  but the diagram on the case implies  the secondary is indeed a proper 50Hz type of device, and there are 'taps' or terminals part way along the secondary coil, so you can either take 24V from the two extreme ends, or 12V from one end and the terminal in the middle,  and as there is a further tapping point from approx 1/3 of the  way along one of the 12V halves, you also have the option of 8V or 4V instead, not useful for your case but it is actually quite a versatile little unit.


Please use your meter to verify the output is AC while we complain to BG that either their data sheet is incorrect or their device labelling...  like they care.


So, assuming that it is the right thing, and  I presume then that you are seeing 10V across the '8V' section of the output winding, as these voltages are all only truly correct supplying the nominal full load, and can be expected to rise a bit off-load, and fall a bit on-load (20-25% variation between on and off load is quite common on small transformers like this - a normal door bell does not give a hoot, and thinner wire keeps the size down).


I suggest you look at the other  pairs of output terminals on the tranny, and select the ends to get  nearer to 24 (or maybe use one end and the middle  '12V' one if it is really more like 16V - 17V, and the '24V setting is a more like 30 ),  then run a pair of wires to the resistor, which  needs to be in-line with one of  the leads, as per the last diagram I linked to in my previous  post.