Just to muddy the waters further, you can also measure the power output (thrust is what we're after) by using EPR, the Engine Pressure Ratio. This is measured by comparing inlet and outlet pressure values over the whole engine. The higher the EPR value, the more oomph (technical term
). On older types (eg Fokker70/10 with RR Tays) at take off we calculate, or lookup, an EPR value to get the required thrust, and type it into the thrust rating panel. On newer types (eg Airbus A320 family) we type in the required assumed outside air temperature, and this will calculate the EPR for us.
On a daily basis we only look at N1 and N2 during engine start. The starter motor is connected to the high pressure spool, and therefore we get a positive reading and increase in N2 first, before the fuel is introduced and light up occurs. After we have a positive reading of N2, N1 will come to life. If it doesn't, then we have a problem (fan frozen to the engine casing? Who did the walkround inspection properley). However N2 is the main rotation parameter we check against other happenings in the start sequence (fuel on, flame ingition, starter motor disengage etc etc).
If however there is a problem with the EPR sensing system we are able to change the engine operation to N1/N2 mode instead and set power requirements accordingly. The 737 crash into the bridge over the Potomac in Washington DC was a prime example of setting a correct EPR, but having incorrect power set due to pressure sensors being iced over. The N1 and N2 values were way to low for the thrust they thought they were getting.
Hope this helps.