Instruments for measuring sub-ohm values are often referred to as low-resistance ohmmeters, milli-ohmmeters, micro-ohmmeters, etc. Some ohmmeters include Kelvin bridges in order to obtain large measurement ranges. The first ohmmeters were based on a type of meter movement known as a 'ratiometer'. Ohmmeters form circuits by themselves, therefore they cannot be used within an assembled circuit.