Together, we estimate, and then measure the frequency of the first three normal modes of the slinky. This is done by counting cycles for 10 seconds. The message is that harmony happens when the ratio of two frequencies is close to a natural fraction involving small numbers. It is common to be off by one cycle, i.e. by counting 34 cycles for mode 3. My spare bow has three weights attached by strings of different length, allowing me to use resonance to cause any single one of them to oscillate using nearly imperceptible hand motion.
Most students are able to guess how it works.
A cello uses resonances to convert string motion into air motion. This device uses resonances to convert hand motion into motion of steel balls. Hold the ruler in you hand and focus on one ball. Using almost no hand motion, try to get that ball to rock back and forth. You will notice that the other two balls are not moving very much.