Dial Indicator

Logan claims amplification. Write down the difference between measuring a size and measuring a deviation — the two need different instruments.

Press the plunger of the dial indicator gently and watch the needle. Estimate how many times larger the needle's travel is than the plunger's.

Cut a 200 mm card strip. Pin it to the board 20 mm from one end with a screw. The long end now moves nine times as far as the short end.

Ratio = long arm divided by short arm. Write it down before you test it, then check it.

Slide a 0.05 mm feeler leaf under the short end. Measure how far the long end rises.

Compare the measured rise with ratio × 0.05 mm. Any shortfall is lost motion in your pivot — the enemy Logan's gearing had to defeat.

Mark a card scale where the long end sweeps, graduated in millimetres. Each millimetre now stands for a fraction of one at the input.

Run every leaf from 0.05 to 0.50 mm through your amplifier, recording the output each time.

Plot the pairs. A good instrument gives a straight line. Curvature means your ratio changes with position — a real and common defect.

Approach one leaf thickness from below, then from above. The two readings differ. That gap is backlash, and it is why indicators are read in one direction only.

Rest the indicator on the surface plate and sweep it across the board's face. Watch the needle wander. You are seeing the board's error, magnified.

Roll the brass rod under the indicator tip. The needle sweep is the rod's out-of-roundness — invisible to the eye, obvious to the dial.

Attempt step 11 with the micrometer instead. It is slow and awkward, because it answers the wrong question. Two instruments, two jobs.

US 283,627, “Improvement in Gages”, John Logan of Waltham, Massachusetts, filed 15 May 1883, granted 21 August 1883.

A date correction. Several respectable secondary sources give this patent as 21 August 1886. The patent document itself is dated 1883, and the number is consistent with 1883 issues. Where a secondary source and the document disagree, the document wins.

What happened to it. Frank Randall, a watchmaker at the E. Howard Watch Company, bought Logan's patent rights in 1896 and went into partnership with Francis Stickney; Randall & Stickney of Waltham then manufactured dial indicators for general industry. The rack-and-pinion arrangement Logan developed is still the mechanism inside most mechanical dial indicators.

Why deviation matters more than size. On a production line nobody re-measures every dimension of every part. They set a master, zero an indicator against it, and then read only the difference for every part after. Reading a difference is faster, needs less skill, and is less error-prone than reading an absolute size — which is what let unskilled operators hold skilled tolerances. The gauge blocks and the comparator later in this batch are both built on the same insight.