You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Nikola_Tesla

Context:
Tesla investigated atmospheric electricity, observing lightning signals via his receivers. He stated that he observed stationary waves during this time. The great distances and the nature of what Tesla was detecting from lightning storms confirmed his belief that the earth had a resonant frequency.

Question:
What kind of electricity was Tesla investigating?

atmospheric