Question: who discovered that electrons have a negative charge
[{'answer': 'Johann Wilhelm Hittorf', 'provenance': [{'wikipedia_id': '9476', 'title': 'Electron', 'start_paragraph_id': 12, 'start_character': 519, 'end_paragraph_id': 12, 'end_character': 541, 'bleu_score': 1.0, 'section': 'Section::::History.:Discovery of free electrons outside matter.\n'}]}]