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Experimental Skills revision notes

Every idea in the Experimental Skills chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 23 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Measuring instruments

Vernier callipers: least count
13× asked, last 2026
LC=1 MSD−1 VSD=MSDN,reading=MSR+n×LC\text{LC}=1\,\text{MSD}-1\,\text{VSD}=\frac{\text{MSD}}{N},\qquad \text{reading}=\text{MSR}+n\times\text{LC}
When NN VSD =(N−1)=(N-1) MSD. If NN VSD =M=M MSD with MM close to NN, LC =∣1−M/N∣=|1-M/N| MSD. nn = coinciding vernier division.
  • Outer jaws give the external diameter, inner jaws the internal diameter, and the thin tail strip the depth of a vessel. All three read MSR + n×+\,n\timesLC and share one zero error.
  • Extended vernier, NN VSD =(kN±1)=(kN\pm1) MSD with k≥2k\ge2: LC =∣k =|k\,MSD−1 -1\,VSD∣=|= MSD/N/N. So 10 VSD =19=19 MSD gives LC =0.1=0.1 MSD, not ∣1−M/N∣=0.9|1-M/N|=0.9 MSD.
  • Not MSD ×N\times N. The vernier constant is MSD divided by NN, a small fraction of one MSD. In an ordinary vernier one VSD is a little shorter than one MSD.
  • Quick method. Convert MSD first: '20 divisions per cm' means MSD =0.5=0.5 mm. Then get VSD in MSD from the coincidence (NN VSD =M=M MSD) and subtract. A stated total of vernier divisions is often a distractor.
Vernier zero error
11× asked, last 2026
true reading=observed−e,e+=+ n×LC\text{true reading}=\text{observed}-e,\qquad e_{+}=+\,n\times\text{LC}
Vernier zero right of main zero: ee positive. Left: ee negative, strictly −(N−n)×-(N-n)\timesLC. Subtract ee with its sign.
010510n=4n=4
Zero to the right: e=+4×0.1=+0.4e=+4\times0.1=+0.4 mm
subtract it
010510n=4n=4
Zero to the left: e=−(10−4)×0.1=−0.6e=-(10-4)\times0.1=-0.6 mm
subtract it (adds 0.6)
  • Negative error: some JEE keys use −n×-n\timesLC instead of the strict −(N−n)×-(N-n)\timesLC. Use the strict rule; if no option matches, try −n×-n\timesLC.
  • Not adding a positive error. With the jaws closed the scale already reads ee too much, so every reading is too large; subtract it. ee is n×n\timesLC, not nn MSD.
Screw gauge: pitch and least count
9× asked, last 2026
pitch=linear distanceno. of rotations,LC=pitchNcircular\text{pitch}=\frac{\text{linear distance}}{\text{no. of rotations}},\qquad \text{LC}=\frac{\text{pitch}}{N_{\text{circular}}}
Reading = MSR + CSR ×\times LC. MSR = linear divisions visible ×\times their value (often 0.5 mm).
  • Backlash error: a worn thread lets the screw turn without moving the spindle. Always reach the final position turning one way only, using the ratchet.
  • Not distance ÷ circular divisions. First get the pitch for one rotation (distance ÷ rotations), then divide by the circular divisions. Watch the decimal: 0.5/100=0.0050.5/100=0.005.
  • Quick method. A reading is a whole multiple of LC, so its last digit is limited: with LC 0.002 cm only even last digits are possible. LC scales as pitch ÷ divisions when either changes.
Screw gauge zero error
12× asked, last 2026
d=MSR+CSR×LC−ed=\text{MSR}+\text{CSR}\times\text{LC}-e
Circular zero nn divisions below the reference line (or division nn on it): e=+ne=+n LC. nn above: e=−ne=-n LC.
  • Not negative for 'below the line'. Circular zero below (ahead of) the reference line means the gauge reads more than zero when closed: positive error, subtract it. Above the line: negative, add its size.
  • Quick method. Read what is asked: a radius is half the corrected diameter; area, volume and density use the corrected value, with significant figures set by the least precise input.
Also remember
  • Maximum error of a product or quotient: add the fractional errors, taking each reading's error as the LC. The smaller quantity, read with the same LC, carries the larger fractional error.
  • Repeated readings: round the uncertainty to one significant digit and the mean to the same decimal place. The LC of the instrument is the smallest step in the last recorded digit.
  • Spherometer: thickness of thin plates and radius of curvature, R=l2/6h+h/2R=l^2/6h+h/2 (ll = leg spacing, hh = height). It cannot measure optical quantities such as specific rotation.

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19 more concepts in 5 topics

Mechanics and properties of matter, Heat and sound, Electricity, Optics and Semiconductor devices. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Mechanics and properties of matter

  • Simple pendulum: g1×
  • Damped pendulum: energy loss
  • Metre scale: principle of moments
  • Young's modulus (Searle's method)
  • Surface tension by capillary rise
  • Viscosity by terminal velocity

Heat and sound

  • Speed of sound: resonance tube
  • Specific heat: method of mixtures

Electricity

  • Metre bridge: resistivity1×
  • Resistance by Ohm's law
  • Galvanometer: half-deflection method2×

Optics

  • Concave mirror and convex lens: u–v method4×
  • Error in focal length3×
  • Convex mirror (using a convex lens)
  • Prism: deviation vs incidence
  • Glass slab: travelling microscope6×

Semiconductor devices

  • p-n junction diode: I–V curves
  • Zener diode: breakdown voltage
  • Identifying components with a multimeter1×

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