This brings in something called Ohm's Law. Let's say you have turned the spigot on only half way, producing some resistance, with a hose attached to fill a pot with water. As you fill the pot with a constant pressure from the spigot you have constant flow rate through the hose. If you open the spigot all the way and double the pressure you are also doubling the flow rate. Electricity works the same way, if we double the voltage we double the current in Amps.
Water resistance flowing through a hose is the same as resistance through a wire.
This resistance is measured in Ohms. The amount of this resistance can be measured by pressure and flow rate or voltage and amps. One Ohm of resistance lets one Amp of current flow when the pressure is one volt. Two volts would give you two amps of current, Six volts would give you 6 amps, etc. You can calculate the amount of Ohms, Volts or Amps with the following equation.