In this article, I discuss manuscript material written by Petrus van Musschenbroek (1692-1761) related to his first experiments with the Leiden jar. Despite the importance of the discovery of the Leiden jar for the history of electricity and the questions that still surround its discovery, a detailed treatment of this manuscript material is lacking in the literature. The main aim of this paper is to provide an outline of the manuscript material and to contextualize van Musschenbroek's first experiments with the Leiden jar. I show how the experiment fits within his research program on electricity and I discuss van Musschenbroek's initial reactions to and analysis of the phenomenon. Before doing so, I first provide a short overview of the treatment of the early history of the Leiden jar in the secondary literature. After that, I discuss van Musschenbroek's treatment of the topic of electricity in the textbooks he published in the years before the discovery of the device. Van Musschenbroek repeatedly emphasized that not enough experimental results were available for an informed theoretical treatment of the phenomenon of electricity to be possible. I then turn to the manuscript material, where I give a general description of the contents of the manuscript and van Musschenbroek's experimental practice. The manuscript material further confirms recent work on the Leiden jar by Silva and Heering, and provides new insights into the way van Musschenbroek himself reacted to the discovery.
Keywords: History of science; Leiden jar; Petrus van Musschenbroek; exploratory experimentation; history of electricity.