What is the maximum allowable current for a 40 ohm, 10W resistor?

What is the maximum allowable current for a 40 ohm, 10W resistor?

Rated current I = √ (P / R) = 0.5A

How to calculate the maximum allowable voltage of 10K Ω 10W resistor
Want a formula

Because P = u ^ 2 / R
So u = PR under root
So under u = root, 1000 Ω * 10W = 100V

To design a shunt circuit, it is required to enlarge the range of 5mA ammeter by 5 times. If the internal resistance of the ammeter is known to be 1K ohm, calculate the shunt resistance

Connect a resistor in parallel with the ammeter, resistance value: 250 Ω (total range is 5 times of the original)
200 euro (5 times larger)

A classmate uses a battery pack, two ammeters, a 12 ohm resistance and a switch to measure the resistance of unknown resistance R. after the classmate designs the circuit, the two ammeters indicate 0.6 and 0.4A respectively, and then judge the resistance of resistance R

If two resistors are in parallel, then: (the switch is installed on the main circuit)
If 0.6 * 12 = 0.4 * r, r = 18
perhaps
0.4 * 12 = 0.6 * r = 8
If two resistors are connected in series, the switch is connected in parallel with one of them
It is 0.4 when the switch is off and 0.6 when the switch is on
If 0.4 * (12 + R) = 0.6 * 12, r = 6
perhaps
If 0.4 * (12 + R) = 0.6 * r, r = 24