Norse mill. In the 13^th century the first so called norse mills were used in Sweden. A simple wooden impeller was placed at the lower end of a vertical shaft. The millstone was placed at the upper end of the shaft. The flowing water was led towards the blades of the turbine wheel through a wooden channel, where its kinetic energy was transferred to the wheel, which consequently started to rotate. It was no longer necessary to mill corn by hand and it became also possible to mill big quantities. A problem with the norse mill was that it had to be buildt close to the watercourse. In order to get a more free location of the mill, the water was sometimes led to the norse wheel from the watercourse through long wooden channels. It was also found out that it was possible to adjust the power of the mill by damming up the water and use headgates and pipes to control the water flow.

The norse mills became a common sight in the Swedish countryside. They were easy to build and easy to operate. No remarkable water head was needed to keep the norse wheel running. It was possible to mill corn even if there was no more water than what could run through a bootleg.

The water wheel was the next step in the hydro power development. Unlike the norse wheel the waterwheel has a horizontal shaft. There were principally two types, undershot wheels for low heads and overshot wheels for higher heads.

In the water wheel the potential energy of the water is transformed to mechanical energy in the rotating shaft. The driving water is filled in highly positioned wheel paddles and is emptied when each paddle reaches its lowest point. The weight of the water makes the wheel rotate. When the water is emptied at the lowest point of the wheel, it has lost all its potential energy in relation to the lower water level.

At higher heads (overshot wheel) the water wheel diameter is about the same as the head. High rates of flow lead to large paddle volume. For these reasons the waterwheels have large dimensions compared to the available power. The rotation speed of the wheel will in most cases be small (4-8 rpm), which requires a large gearing. The big dimensions and the low rotation speed are the main reasons why the water wheel has been abandonned now.

It did not take long time before ingenious people realized that the water wheel could be used to many different things, besides corn-milling. The hydro power gave birth to many different inventions. One can say that it was as important for the technology of these days as the steam engine and electricity became later on. It is no overstatement to say that the water wheel is one of the most important inventions for mankind.

The rod transmission. In the Middle Ages ore-mining started in Bergslagen and Dalarna. To be able to run the heavy machines used for ore-mining, the mining operations had to be located near flowing water. This was a great problem, as the ore supply could not be completely utilized. Many engineers were busy trying to find a solution of the problem and the Swede Christopher Polhem finally found out a brilliant solution. He invented the wooden rod transmission. The design was rather complicated, the rotating movement was transferred from the whirling water wheel through shaft cranks to bars which moved backwards and forwards. The bars were suspanded on poles and could be coupled together. Such  chains of transmission rods could convey mechanical energy over conciderable distances.

Now it became possible to drive elevators, hammers and other things in the mines which were too heavy for the workers to manage. A shortage with the rod transmission was that it was exposed to great stresses and required much maintenance and a rather big part of the power was lost because of  friction in links and joints.

Electrical power. Many people thought that the steam engine should outrival the hydro power, but it turned out to be wrong. Instead technical development of the water wheel started. The wooden wheels were replaced by metal ones. By using belts and steel wires the power could be transferred in new and much more efficient ways.The big change however did not happen unttil the electricity became common.

The change was possible  thanks to the electric generator which was invented in 1849. About 30 years later it had been developed into a practically functioning machine for production of electricity. The first Swedish hydro power plant with electricity generation was put into operation in 1882. Twenty years later practically all water turbines were used for electric energy production.

The importance of electrical power access. Thanks to hydro power with electrical generation and also our technology for electrical power transmission , we have dramatically increased both the amount of energy and its accessibility. The result is that the electrical power in many connections has replaced older kinds of energy. An example is the railway traffic, where electricall locomotives have replaced steam-operated ones.

In the 20^th century we have also got a great number of entirely new electrical power applications, comprising everything from higjly energy demanding industrial processes to small "necessary" apparatus at home (like electrical toothbrushes).