How do you maintain a constant load on the cpu?
Install a terminal emulator and try the following:

dd if=/dev/urandom of=/dev/null

or

dd if=/dev/random of=/dev/null

It will write random values to the nirvana, should cause a constant load. But since you typically have a lot of different processes on your phone running, it is hard to assure some processes don't wake up and cause load interruptions. Therefore, switch the phone to flight mode, disable all other connections like wifi/bluetooth/nfc etc. and try to shut down as many services as you can. But still, e.g. the garbage collector could run at unexpected times. Android is just not a realtime OS, but for the measurement you are probably attempting it should be sufficient.

And in case your device has several cores: Open more tabs in the emulator and run more dd processes.