Emerson Electric’s $27.6 billion offer for Rockwell Automation is as logical as it was easy to rebuff. The combination would give Emerson heft in what has fast become one of the buzzwords of the industry: process automation, namely making sensors and software that help devices chat to one another, and manufacturers do things better and faster. The trouble is that Emerson is offering too little of the wrong currency. The bid Rockwell’s board publicly rejected on Tuesday would have left its shareholders with just over a quarter of the enlarged group, and a $13.8 billion slug of cash. But to