Help needed solve fire house problem 24v to 12v 100 milli amp
Since you state you are going to use a resistor, I'll directly answer you about the current limiting question you asked.

If the bell works properly at \$12V\$ and it actually draws \$100mA\$ when attached to a \$12V\$ source, then you actually can use a resistor and it will still only draw the \$0.1A\$ and the bell will still only consume \$1.2W\$, as before.

To drop \$24V\$ down to \$12V\$ for the bell you would use a \$\frac{24V-12V}{100mA} = 12\Omega\$ resistor. However, you will need to also dissipate another \$1.2W\$ in the resistor. So I'd recommend getting a \$5W\$ resistor for this purpose (better safe than sorry.) \$10W\$ resistors are also available and may not be that much more expensive.

There are other ways, but given you are only wasting an extra \$1.2W\$, you may be right and there may be less incentive to make things overly 'difficult' here. The risks are modest, as you are dealing with low voltages and low wattage. However, this is an antique device, so you will want to be sure about your estimate of the required current. The resistor makes a poor voltage regulator. But it is easy and may work fine. I'd recommend verifying the current required at \$12V\$ before using a resistor, if possible, and certainly double checking the voltage across the bell afterwards.

So, in this circumstance the resistor itself doesn't do all the current limiting work. It does exactly half of it. Your bell does the other half. The \$10A\$ rating for your power source is almost certainly just a compliance rating and means it can handle up to that amount, but not more.

The risk you are taking here is that the bell's rated current draw is assumed to be known, fixed and unvarying. But it is a bell. And the current may vary. If so, the resistor does a very poor job regulating the voltage and you may expose the bell to a varying voltage exceeding what it can well tolerate. This is probably why you may want to consider other options offered here.