The long-sought after memristor--the "missing link" in electronic circuit theory--has been invented by Hewlett-Packard Laboratories. As the fourth passive component type--after resistors, capacitors and inductors---memristors will enable HPs ultra-high-density crossbar switch (pictured here) to pack a record 100 Gbits onto a single die, compared with 16 Gbits for the highest density flash memory chips today. HP's crossbar switch uses tiny nanowires instead of transistors to address memory cells, thereby side-stepping the scaling problem with transistors by eliminating their need in bit cells. HP has been experimenting with organ compounds to enable the crossbar, but has now concluded that it needed the missing passive component called a memristor to make its crossbar work. Memristors serve as the memory element in bit cells, by storing information in their changing resistance instead of as charge. As the missing link passive component, memristors share characteristics with each, but will require that virtually every electronics textbook today to be rewritten to account for their unique behaviors. A University of California professor used mathematics to deduce the existence of this fourth circuit element type back in 1971. Now HP claims to have discovered the first instance of a memristor, which it created with a bi-level titanium dioxide thin-film that changes its resistance when current passes through it.