The hypothesis that a higher DC voltage applied to the electrolysis process will increase the rate at which the hydrogen gas is produced has been proven to be true.
The second hypothesis, that the rate of production of hydrogen gas will increase as the concentration of salt increases, is not proven to be true.
Electroplating is an industrial process which is used to produce a coating on metal surfaces. Electrolysis is also used in measuring devices such as the pH meter, while astronauts produce oxygen in space through the electrolysis.
This science fair project may also be repeated by using different types of electrolytes such as vinegar or sulfuric acid.
Also, examine the effects of raising or lowering the temperature of the solution during the process of electrolysis.
Electrolysis - http://en.wikipedia.org/wiki/Electrolysis
Electrolysis - http://www.tutorvista.com/content/chemistry/chemistry-iii/redox-reactions/electrolysis.php
Use only batteries. Do not connect the wires to a mains supply.