Take the load off and look at where the pointer sits.
01Before anything else, put the instrument in the state that ought to read zero.Nothing on the hook for a balance, jaws shut for a calliper, melting ice for a thermometer, circuit disconnected for a meter. Every instrument has such a state, and that’s where its error shows.
02Read what it says there, with the same care you would give a real measurement.The zero reading is a real measurement. Read it square on and to the instrument’s own precision: a carelessly read zero error puts its own error into everything you correct with it.
03Write it down with its sign, on the page, before you touch the experiment.Carried in your head it turns into "about a quarter" and its sign goes missing. The sign is half the information.
04Take your measurement normally, and record what the instrument actually says.Record the OBSERVED value, not a value you have already corrected in your head. The examiner wants to see both, and correcting silently means an arithmetic slip can’t be traced.
05Correct it: true = observed − zero error.The same subtraction whichever way the error goes. Subtracting a negative error adds it back, and that’s ordinary arithmetic rather than a rule to remember separately.
06Check the zero again at the end if the experiment was a long one.Springs creep and meters drift. If the zero has moved during the experiment, you have found something important about how much your results are worth.
Laboratory
You haven’t seen what this instrument reads with nothing on it. Take the load off and look before you write anything down: whatever it reads at rest is carried into every measurement you take with it.