An adiabatic differential scanning calorimeter has been built to conduct calorimetric measurements on plasmas. The calorimeter consists of two identical cylindrical cells, which can be operated individually. Heat losses to the ambient are minimised. The cells can be heated by a plasma or by ohmic heating. For the latter, a resistor is installed in each cell. The accuracy of the calorimeter is analysed resulting in different parameters. The absolute accuracy of a repeated experiment is 0.06 W for each cell. The relative reproducibility (difference in power between the cells relatively to the input power, when repeating measurements) is found to be within 0.01 W. The motivation for the research is the publication of peculiar measurements in specific mixtures containing hydrogen by, mainly, Mills. These measurements include excess power generation and excessive broadened hydrogen lines indicating temperatures from tens to hundreds of eV. Calorimetric measurements are done on DC discharge plasmas at 1.5 mbar. The used gases are mixtures of hydrogen in argon or in helium. A part of the measurements have been done with metallic pieces of strontium in the cell. Power input of the cells is compared between different plasmas and between heating by plasmas and resistors. The design of the calorimeter allows spectroscopic measurements in the visible part of the spectrum. Besides the gases used for the calorimetric measurements, also pure hydrogen and mixtures of various percentages of hydrogen in neon and xenon are used. The Balmer lines of hydrogen are recorded and found to consist of two or three separate Gaussian profiles, from which an upper Doppler temperature is derived. Excessive broadening of the Balmer lines is found in all used gas mixtures as well as in pure hydrogen. The derived temperature of the hottest hydrogen atoms is in the range of 24-73 eV, while all non-hydrogen atoms in the discharge remain cold (0.1 eV). Additionally, the influence of the pressure and burning voltage of the plasma on the temperature of the hydrogen atoms is investigated. The pressure dependency is investigated in pure hydrogen and argon/hydrogen mixture in the range of 0.8-9.0 mbar. It is found that the amount of broadening is not dependent on pressure in the argon/hydrogen case but it decreases with rising pressure in pure hydrogen. The burning voltage dependency is investigated in a helium/hydrogen mixture, in the range of 173-364 V. The temperature of the hottest hydrogen atoms increases with increasing burning voltage, while the temperature of the colder hydrogen atoms remains constant. The excess power generation, predicted by Mills, is not reproduced. The excessive broadening of the Balmer lines, conversely, is. It is shown that the presence of hot hydrogen atoms does not necessarily have to lead to excess power generation.