Point at or tap any part to see its name. Pick a voltage to run the test at that voltage.
In later pictures, amber dots show current, the flow of electricity. Here each blue dot is one electron.
Each atom center is drawn as one red ball. The number on it is how many protons it holds, such as +6 for carbon. The center also holds neutrons, which have no charge.
The rings, also called shells, are a simple model of an atom. Real electrons do not travel in neat circles. The drawing is not to scale.
A bond is a link between two atoms that share electrons. In rubber and glass, each line between two atoms is a bond holding two shared electrons, one from each atom.
Oxygen and nitrogen also keep some outer electrons on their own atom, outside any bond. They hold those tight too.
In air, one line stands for a double or a triple bond. The two atoms of a nitrogen molecule share six electrons. The two atoms of an oxygen molecule share four.
An insulator lets almost no current through. It is never exactly zero. A conductor, like copper, lets current through easily.
The rubber is EPR, a rubber used to insulate wire. It is long chains of carbon and hydrogen, with a short side branch here and there.
Glass is mostly silicon and oxygen. Window glass also holds some sodium and calcium, which are not drawn. Real glass is not a neat grid.
Air is mostly nitrogen and oxygen molecules. A molecule is a few atoms bonded together. Real air molecules are much farther apart than drawn.
The lean of the bond electrons is a simple picture. Voltage pulls electrons a tiny amount toward plus and atom centers a tiny amount toward minus. The drawing shows it much larger than it really is.
Electrons torn loose move from negative to positive. Later pictures draw current going the other way, from positive to negative. Both describe the same current.
The breakdown figures are typical values for a piece 1 millimeter thick at room temperature. Real pieces vary with the mix, thickness, heat, and moisture.
Rubber breaks down at about 20,000 volts. Rubbers used on wire test from about 16,000 to 37,000 volts per millimeter.
Window glass breaks down at about 10,000 volts. Tests give 9,800 to 13,800 volts per millimeter.
Air breaks down at about 3,000 volts, the usual figure for air. A gap as small as 1 millimeter holds somewhat more. Damp air, dust, sharp points, and high altitude make air break down sooner.
RHW is a rubber-covered building wire. In small sizes it has about 1.1 millimeters of insulation and is rated for 600 volts. THHN, the wire apprentices pull most, has thinner plastic insulation with a nylon skin.
New insulation can take far more than its rating, but that extra room is no excuse. Heat, age, nicks, and water all lower what insulation can take. Insulation that is nicked, burned, or wet gets replaced.
A burned path in rubber usually turns to carbon, which conducts. A hole in glass usually fills with air, so it breaks down at about the figure for air.
After a test, the piece can hold a charge even with the test set off. It is grounded first, then tested before anyone touches it.
Air heals once the spark stops. The spark still burns the metal. An arc that keeps going can start a fire and badly burn anyone near it.
This test set shuts itself off the moment current flows. A real circuit has no such limit. A short or a breakdown on a building circuit lets a large current flow until a breaker or fuse opens.
High-voltage testing is a trained task. No one stands near the piece while a test runs. Every voltage here can shock or kill.
Insulation can be damaged where you cannot see it. Never trust insulation to keep you safe from a live part. Test before you touch, every time.
Skin is not an insulator. It is a poor conductor, so current can flow through a person.