51839
In 1873 , Frederick Guthrie observed that a grounded , white hot metal ball brought in close proximity to an electroscope would discharge a positively charged electroscope , but not a negatively charged electroscope .
 Frederick Guthrie
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Frederick Guthrie