A thin-walled non-airtight box of total mass $M$ rests on a calibrated scale, reading $Mg$ in still surrounding air at temperature $T_0$. Placed alone on the scale, a small electric heater of mass $m$ reads $mg$, with $M > m$. The heater is now set inside the box and switched on. After a long time the system reaches steady state: the heater glows at constant temperature, the air filling the box settles at a uniform temperature $T_h > T_0$, and cool air slowly enters through small vents near the bottom while warm air exits through small vents near the top. The scale now reads $W$. Which of the following is true?