What to measure, and what it means
Every number below goes into your notebook and then the Experiment Data Log. The analysis code uses them to calculate stress, strain, work, power and efficiency.
What to record
| Measurement | Procedure step | How to get it |
|---|---|---|
| Unstretched length, width and sample mass | 1 | Calipers and precision scale, with the sample resting on the razor blade |
| Unstretched thickness | 1 | Thickness gauge, sample off the blade |
| Stretched thickness and hanging mass | 1 | Under the hanging mass, before coating |
| Thin-wire resistance | 2 | Multimeter across the wire only (about 1 Ω) |
| Active length | 3 | Between the clips, sample taut but not stretched |
| Total series resistance | 3 | Multimeter, target 1.9–2.1 Ω |
| Liquid-metal connection resistance | 3 | Total series resistance − thin-wire resistance |
| Voltage, frequency, duty cycle and duration | 5 | Power supply and WaveForms settings |
| Average steady-state current | 5 | Power supply display; for tests above 1 Hz, use a single 0.1 Hz cycle run beforehand |
| Post-test liquid-metal resistance | 5 | Multimeter; an increase shows the connection is degrading |
| LED start frame, stretched width and stretched length | 6 | From the MATLAB analysis (video-measured) |
Contraction strain
L₀ is the distance between the clips in the first tracked frame, when the sample is fully stretched under its hanging mass. L(t) is that distance at any later moment. Don’t confuse L₀ with the unstretched length you measured before mounting.
For example, shortening from 30 mm to 27 mm is 3 mm of contraction, or 10% strain. Positive values mean the sample has contracted. Within one cycle, the maximum strain minus the minimum strain is the peak-to-peak strain, the stroke the sample delivers each cycle.
Measuring current
The power supply display updates too slowly to follow fast switching, so above 1 Hz its reading can’t be trusted. For faster tests, run one 0.1 Hz cycle before the experiment and use that steady-state current. If the current falls by more than about 50 mA during a test, the connection is degrading.
Optional: oscilloscope check
The protocol doesn’t require an oscilloscope, but it’s a quick way to confirm the MOSFET is switching. For a standard oscilloscope with shared, earth-referenced probe grounds, connect it as shown. Check your instrument’s grounding first.
| Probe | Tip connection | Ground connection | Expected signal |
|---|---|---|---|
| CH1 | MOSFET gate | Common return / source | About 0–5 V square wave |
| CH2 | MOSFET drain | Same common return / source | Near 0 V when on; near the supply voltage when off |
Never attach a grounded probe clip to the drain or the supply positive; that can short the circuit through the oscilloscope. Differential or isolated measurements need a suitably configured instrument.
The gate trace confirms the command and the drain trace confirms the switching. Neither one measures the current through the sample.