Electrical detection is used for the first time to achieve real-time operation of a low pressure cascade impactor. An instrument is developed using Berner type multijet low pressure impactor stages with minimum modification. The cut-sizes of the seven channel system range from 0.030 to 1.0 μm. A multichannel electrometer is constructed using low cost monolithic electrometer operational amplifiers. The zero-check technique is applied to achieve a lowest detectable current of 10 fA, less than a tenth of the input bias current of the operational amplifiers. The time resolution is 1 min in the lock-in mode and 1 s in the high concentration mode. The sensitivity and size resolution of the instrument are comparable to the corresponding values of the Electrical Aerosol Analyzer. The feasibility of the instrument in measuring submicron number size distribution has been tested by comparison measurements with a differential mobility analyzer. The results show quantitative agreement.