## Experiment 1-7

Sensitivity and Precision

### Background Information

The more sensitive a balance is, the smaller variation in mass the balance can detect and measure.  But no balance is prefect.  Every balance has its limitations.  A real mass smaller than the sensitivity of a balance will not be detectable using the balance.  The sensitivity of a balance depends on several design characteristics as well as the patience of the experimenter.  For example, the more stable the design of a balance, the more mass is needed to move the balance noticeably.  And the more friction at the locations where movement is required, the less sensitive the balance will be.

A related property is the precision of the balance.  The precision is determined by independently measuring the same object repeatedly and noting the spread of the measured results.

### Procedures

Measure the precision of your balance.  To do this,
1. select three or more objects of significantly different masses.
2. Measure the mass of each in succession as carefully as possible.  Trying to forget the earlier measurements to avoid prejudice, remeasure the same masses again several times, always measuring another object between measuring the same object.
3. Average the measured masses for each object.
4. Calculate the difference of each measurement from the average for that object.  The average difference for the measurements is a measure of the precision.  Note the balance may have a different precisions for different masses.
Determine the sensitivity of your balance.  To do this,
1. find the mass of a section of contiguous squares from a sheet of graph paper.  (If a single sheet has no apparent mass, change the following directions to measure multiple sheets of paper.)
2. Calculate what should be the mass of a single square of graph paper for later use.
3. Cut off smaller sections of squares and place successively smaller pieces on the balance to determine where the limit is between what mass does cause a movement of the balance and smaller pieces that cause no detectable motion.  It is advisable to try slightly larger pieces that those too small and slightly smaller than those barely detectable until the borderline is determined.  Count the number of graph squares that cause the smallest detectable motion.
4. Multiply that number of squares times the calculated mass of a single square to determine the balance's sensitivity.

Record your results in your science journal and write a formal report to earn credit.

created 6/30/2002
revised 11/20/2003
by D Trapp