Early 20th Century Article on Wind Turbine Efficiency

I found an interesting article through one of The Economist articles (Trade winds). The author is Albert Betz, and he calculated the theoretic limit of windmill efficiency in the early 20th century ("Das Maximum der theoretisch möglichen Ausnutzung des Windes durch Windmotoren (Theoretical limit for best utilization of wind by wind-motors)"). The article itself is hard to find, and if anyone can help me locate it, I would really appreciate it. Anyhow, this article gets cited by about 50 papers. Some of the seem very interesting, and I will definitely have a look at them.