Commutator (electric) A commutator is a moving part of a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine.
The word equation that would represent the formation of lithium oxide from lithium metal and oxygen is: lithium + oxygen to lithium oxide. In chemical symbols, the balanced chemical equation is expressed as: Li + O2 = Li2O.
Magnesium metal reacts with the oxygen (O2) of the air to form magnesium oxide. Mg + O2  magnesium oxide. After the magnesium sample has reacted completely, you determine the mass of magnesium oxide product. From these two masses, you calculate the percentage composition of magnesium oxide.