When an alternating current is passed through a Growler, it sets up a magnetic flux in the iron of the armature or stator spanned by the jaws of the Growler.
As this flux passes through any coil, it induces a potential. A current will flow if the coil is short-circuited. When current flows, it sets up a magnetic field around the shorted coil which can be detected with an iron feeler. (The increased load on the Growler sometimes changes the tone of the hum; hence the name "Growler".) In many cases a meter can be used to measure a change in magnetic flux, or to measure the increased current requirements of the Growler
The most common way of using a Growler is the "feeler method" in which the Growler spans a slot containing a coil, and a "feeler" or iron, such as a hack-saw blade is held about 1/4" above the slot containing the other side of the same coil
If the coil is shorted the feeler will be pulled down to the slot and will stick and vibrate. The action is very positive and is recognized instantly.
The feeler can also be used on the same side of the coil that is spanned by the Growler, either a separate feeler or the convenient built-in feeler.
Open circuits can be detected by shorting adjacent commutator bars with a screw driver, or any other piece of metal. Good coils will spark as the bars are shorted. No sparks indicate the coil is open.
Growlers will work on any armature where the coil can be isolated magnetically. Typically these are found in DC armatures, though some AC rotors, depending on how they are wound, may also be tested. In an AC motor, if there is a squirrel cage or other type of ring that prevents individual coils from being isolated magnetically, then this may interfere with a growler’s ability to function.