An electric sad iron heats its pressing plate with an internal resistance element. The cutaway shows an early arrangement of a hollow metal shell, a raised handle and a flat insulated heater next to the soleplate. The calculator compares electrical power and an ideal plate warm-up estimate using an explicit material heat capacity.
Electric Sad Iron Interactive Calculator
Explore an early electric iron cutaway and ideal plate heat-up. Adjust voltage, resistance, plate mass, temperature rise and the material heat-capacity assumption.
Equation Used
- One uniform-temperature plate with supplied constant specific heat.
- Constant resistance and continuous applied voltage.
- No air losses, fabric heat transfer or other thermal masses.
- No thermostat, element-temperature or electrical-installation model.
Ideal plate-only heat-up from 20°C. No thermal losses, fabric contact or thermostat model.
Heat travels through an insulated element into the plate
The illustration follows the construction principle in an early electric sad-iron patent: a shaped metal body, handle supports, a bottom pressing plate and a flat resistance element held between insulating layers. The lower view removes the covers to show a continuous ribbon on a shaped insulating support.
The insulation separates the electrically active element from the surrounding metal while allowing heat transfer. The parts remain stationary during the replay. The changing plate colour and downward arrows represent heating, not visible incandescence, mechanical motion or an electrical-current path.
The main scene is a side cutaway; the second view is a heater plan. Layer thicknesses and ribbon spacing are exaggerated for visibility. They are not dimensions for building or rewiring an iron.
The depicted early-style arrangement has no modeled thermostat. Later irons can include temperature control, but a thermostat cannot be assumed to be present in every historical design. The replay starts over for explanation; it does not claim the plate physically cools instantly.
Explore electrical power and thermal mass
Applied voltage and element resistance determine electrical power and current. Plate mass, the requested temperature rise and specific heat determine the ideal heat needed by the modeled plate.
The default specific heat is 900 J/(kg·K), an illustrative aluminum value. Enter a value appropriate to the plate material and temperature interval. The cited physics reference gives 452 J/(kg·K) for iron or steel as a comparison value; actual material properties vary with composition and temperature.
These inputs are calculation scenarios, not supply-voltage settings or product ratings. The calculator does not establish electrical compatibility, a usable pressing temperature or fabric performance.
Electrical power and ideal warm-up energy
For a constant, purely resistive element, I=V/R and P=V²/R. Use RMS voltage for sinusoidal AC. Plate heat is Q=mcΔT and the ideal warm-up time is t=Q/P; divide seconds by 60 for minutes.
The model assumes all element power becomes stored heat in one uniform-temperature plate. It excludes heat lost to air, heat stored in the shell and handle, heat transferred to fabric and any thermostat cycling. It therefore does not predict the measured warm-up of a complete appliance.
The specific-heat input is constant across the entered temperature interval. Resistance is also held constant, even though real material properties can change during heating. Electrical power in Btu/h is a unit conversion using 1 W≈3.41214 Btu/h.
Default calculation
At 120 V and 9.6 Ω, the element draws 12.5 A and dissipates 1500 W. For a 1.5 kg modeled plate, c=900 J/(kg·K) and a 170 K rise, the energy is 229500 J and the ideal time is 153 seconds, or 2.55 minutes.
At the same electrical conditions, changing only c to 452 J/(kg·K) gives 115260 J and 76.84 seconds. This compares thermal properties in the ideal model; it does not assert that a real iron or steel appliance heats in that time.
Doubling plate mass doubles the ideal time. Doubling resistance halves power and doubles time at unchanged voltage. Increasing voltage changes power quadratically. None of these mathematical comparisons is an instruction to operate an appliance outside its rating.
Garment throughput, component life and practical warm-up depend on the complete appliance and its operating conditions.
Limits of the ideal plate model
Electrical power alone does not determine pressing quality or thermostat behavior. Heat-transfer contact, surface losses, material properties and the rest of the appliance affect actual performance.
The drawing provides a recognizable construction explanation while the calculator solves an ideal lumped thermal model. It does not calculate local ribbon temperature, insulation temperature, current leakage, fabric damage, element lifespan or circuit capacity.
Start and target temperatures are displayed as 20°C and 20°C plus the requested rise. Those values are illustrative calculation endpoints, not recommended garment settings.
Electric sad iron questions
Why is there no moving linkage?
The operating process is resistance heating. The animation shows the heating progress and layered construction while the physical parts stay still.
Why can I change specific heat?
Specific heat varies by material. Entering it lets you compare the heat needed by the plate material being studied.
Is the displayed time a prediction for my iron?
No. It assumes no heat loss and ignores the shell, handle, fabric and temperature control.
Is the colour a prediction of glowing metal?
No. It is a visual indicator of the calculated uniform plate temperature.
Does changing voltage show an allowed operating setting?
No. The voltage input explores a mathematical relationship and does not replace the equipment rating.
References
- J. W. Phelps, Electric Sad-Iron, US966703A — side elevation, section, shaped resistance element and insulating layers; published 1910.
- Michigan State University PHY231: specific heat and thermal energy — Q=mcΔT and example specific heats for aluminum and iron/steel.
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