In 1900, at the close of the first decade in which electric systems had become a practical alternative for manufacturers, less than 5 percent of the power used in factories came from electricity. But the technological advances of suppliers made electric systems and electric motors ever more affordable and reliable, and the suppliers' intensive marketing programs also sped the adoption of the new technology. Further accelerating the shift was the rapid expansion in the number of skilled electrical engineers, who provided the expertise needed to install and run the new systems. In short order, electric power had gone from exotic to commonplace. But one thing didn't change. Factories continued to build their own power-supply systems on their own premises. Few manufacturers considered buying electricity from the small central stations. Designed to supply lighting to local homes and shops, the central stations had neither the size nor the skill to serve the needs of big factories. And the factory owners, having always supplied their own power, were loath to assign such a critical function to an outsider. They knew that a glitch in power supply would bring their operations to a halt ─ and that a lot of glitches might well mean bankruptcy. As the new century began, a survey found that there were already 50,000 private electric plants in operation, far surpassing the 3,600 central stations.